This article is dedicated to the memory of Dennis Ritchie (1941-2011).

Please note that BSD manpages are usually better as compare to Linux.

#1: Red Hat Enterprise Linux

RHEL is developed by Red Hat and targeted toward the commercial market. It has one of the best documentations covering basis of RHEL to advanced topics like security, SELinux, virtualization, directory server, clustering, JBOSS, HPC, and much more. Red Hat documentation has been translated into twenty-two languages and is available in multi-page HTML, single-page HTML, PDF, and EPUB formats. The good news is you can use the same documentation for CentOS or Scientific Linux (community enterprise distros). All of these documents ship with the OS, so if you don't have a network connection, then you have them there as well. The RHEL docs covers everything from installation to configuring clusters. The only downside is you need to be a paid customer. This is perfect for an enterprise company.

1. RHEL Documentation: Unknown format
2. Support forums: Only available to Red Hat customer portal to submit a support case.

A Note About CentOS Wiki and Forums

CentOS (Community ENTerprise Operating System) is a free rebuild of source packages freely available from a RHEL. It provides truly reliable, free enterprise Linux for personal and other usage. You will get RHEL stability without the cost of certification and support. CentOS wiki divided into Howtos, Tips & Tricks, and much more at the following locations:

1. Documentation Wiki
2. Support forum

#2: Arch Wiki and Forums

Arch Linux is an independently developed, Linux operating system and it comes with pretty good documentation in form of wiki based site. It is developed collaboratively by a community of Arch users, allowing any user to add and edit content. The articles are divided into various categories like networking, optimization, package management, system administration, X window system, and getting & installing Arch Linux. The official forums are useful for solving many issues. It has total 40k+ registered users with over 1 million posts. The wiki contains some general information that can also apply in other Linux distros.

1. Arch community Documentation: Wiki format
2. Support forums: Yes

#3: Gentoo Linux Wiki and Forums

Gentoo Linux is based on the Portage package management system. The Gentoo user compiles the source code locally according to their chosen configuration. The majority of users have configurations and sets of installed programs which are unique to themselves. The Gentoo give you some explanation about the Gentoo Linux and answer most of your questions regarding installations, packages, networking, and much more. Gentoo has very helpful forumwith over one hundred thirty-four thousand plus users who have posted a total of 5442416 articles.

1. Gentoo community documentation: Handbook and Wiki format
2. Support forums: Yes
3. User-supplied documentation available at gentoo-wiki.com

#4: Ubuntu Wiki and Documentation

Ubuntu is one of the leading desktop and laptop distro. The official documentation developed and maintained by the Ubuntu Documentation Project. You can access a wealth of information including a getting started Guide. The best part is information contained herein may also work with other Debian-based systems. You will also find the community documentation for Ubuntu created by its users. This is a reference for Ubuntu-related 'Howtos, Tips, Tricks, and Hacks'. Ubuntu Linux has one of the biggest Linux communities on the web. It offers help to the both new and experienced users.

1. Ubuntu community documentation: wiki format.
2. Ubuntu official documentation: wiki format.
3. Support forums: Yes.

#5: IBM Developer Works

IBM developer works offers technical resources for Linux programmers and system administrators. It contains hundreds of articles, tutorials, and tips to help developers with Linux programming and application development, as well as Linux system administration.

1. IBM Developer Works Documentation: Unknown format
2. Support forums: Yes.

#6: FreeBSD Documentation and Handbook

The FreeBSD handbook is created by the FreeBSD Documentation Project. It describes the installation, administration and day-to-day use of the FreeBSD OS. BSD manpages are usually better as compare to GNU/Linux man pages. The FreeBSD comes with all the documents with upto date man pages. The FreeBSD Handbook covers everything. The handbook contains some general Unix information that can also apply in other Linux distros. The official FreeBSD forums also provides helps whenever you will get stuck with problems.

1. FreeBSD Documentation: Unknown format
2. Support forums: Yes.

#7: Bash Hackers Wiki

This is an excellent resource for bash user. The bash hackers wiki is intended to hold documentations of any kind about the GNU Bash. The main motivation was to provide human-readable documentation and information to not force users to read every bit of the Bash manpage - which is hard sometimes. The wiki is divided into various sections such as - scripting and general information, howtos, coding style, bash syntax, and much more.

1. Bash hackers wiki in wiki format

#8: Bash FAQ

A wiki designed for new bash users. It has good collections to frequently asked questions on channel #bash on the freenode IRC network. These answers are contributed by the regular members of the channel. Don't forget to check out common mistakes made by Bash programmers, in BashPitfalls section. The answers given in this FAQ may be slanted toward Bash, or they may be slanted toward the lowest common denominator Bourne shell, depending on who wrote the answer. In most cases, an effort is made to provide both a portable (Bourne) and an efficient (Bash, where appropriate) answer.

1. Bash FAQ in wiki format.

#9: Howtoforge - Linux Tutorials

Fellow blogger Falko has some great stuff over at How-To Forge. The site provides Linux tutorials about various topic including its famous "The Perfect Server" series. The site is divided into various topics such as web-server, Linux distros, DNS servers, Virtualization, High-availability, Email and anti-spam, FTP servers, programming topics, and much more. The site is also available in German language.

1. Howtoforge in html format.
2. Support forums: Yes

#10: OpenBSD FAQ and Documentation

OpenBSD is another Unix-like computer operating system based on Berkeley Software Distribution (BSD). It was forked from NetBSD by project. The OpenBSD is well known for the quality code, documentation, uncompromising position on software licensing, with strong focus on security. The documenation is divided into various topics such as - installations, package management, firewall setup, user management, networking, disk / RAID management and much more.

1. OpenBSD in html format.
2. Support forums: No, but mail lists are available.

#11: Calomel - Open Source Research and Reference

This amazing site dedicated to documenting open source software, and programs with special focus on OpenBSD. This is one of the cleanest and easy to to navigate website, with focus on the quality content. The site is divided into various server topic such as DNS, OpeBSD, security, web-server, Samba file server, various tools, and much more.

1. Calomel Org in html format.
2. Support forums: No

#12: Slackware Book Project

Slackware Linux was my first distro. It was one of the earliest distro based on the Linux kernel and is the oldest currently being maintained. The distro is targeted towards power users with strong focus on stability. Slackware is one of few the most "Unix-like" Linux distribution. The official slackware book is designed to get you started with the Slackware Linux operating system. It's not meant to cover every single aspect of the distribution, but rather to show what it is capable of and give you a basic working knowledge of the system. The book is divided into various topics such as Installation, Network & System Configuration, System administration, Package management, and much more.

1. Slackware Linux books in html, pdf, and other format.
2. Support forums: Yes

#13: The Linux Documentation Project (TLDP)

The Linux Documentation Project is working towards developing free, high quality documentation for the Linux operating system. The site is created and maintained by volunteers. The site is divided into subject-specific help, longer and in-depth guide books, and much more. I recommend this document which is both a tutorial and a reference on shell scripting with Bash. The single list of HOWTOs is also a good starting point for new users.

1. The Linux documentation project available in multiple formats.
2. Support forums: No

#14: Linux Home Networking

Linux home networking is another good resource for learning Linux. This site covers topics needed for Linux software certification exams, such as the RHCE, and many computer training courses. The site is divided into various topics such as networking, samba file server, wirless networking, web-server, and much more.

1. Linux home networking available in html and PDF (with small fee) formats.
2. Support forums: Yes

#15: Linux Action Show

Linux Action Show ("LAS") is a podcast about Linux. The show is hosted by Bryan Lunduke, Allan Jude, and Chris Fisher. It covers the latest news in the FOSS world. The show reviews various apps and Linux distros. Sometime an interview with a major personal in the open source world is posted on the show.

1. Linux action show available in audio/video format.
2. Support forums: Yes

#16: Commandlinefu

Commandlinefu lists various shell commands that you may find interesting and useful. All commands can be commented on, discussed and voted up or down. Ths is an awesome resource for all Unix command line users. Don't forget to checkout all top voted commands here.

1. Commandlinefu available in html format.
2. Support forums: No

#17: Debian Administration Tips and Resources

This site covers topics, tips, and tutorial only related to Debian GNU/Linux. It contain interesting and useful information related to the System Administration. You can contribute an article, tip, or question here. Don't forget to checkout top articles posted in the hall of fame section.

1. Debian administration available in html format.
2. Support forums: No

#18: Catonmat - Sed, Awk, Perl Tutorials

This site run by a fellow blogger Peteris Krumins. The main focus is on command line and Unix programming topics such as sed, perl, awk, and others. Don't forget to check out introduction to sed, sed one liner explained, the definitive guide to Bash Command line history, and awk liner explained.

1. catonmat available in html format.
2. Support forums: No

#19: Debian GNU/Linux Documentation and Wiki

Debian is another Linux based operating system that primarily uses software released under the GNU General Public. Debian is well known for strict adherence to the philosophies of Unix and free software. It is also one of popular and influential Linux distribution. It is also used as a base for many other distributions such as Ubuntu and others. The Debian project provides its users with proper documentation in an easily accessible form. The site is divided into wiki, installation guide, faqs, and support forum.

1. Debian GNU/Linux documentation available in html and other format.
2. Debian GNU/Linux wiki
3. Support forums: Yes

#20: Linux Sea

The book "Linux Sea" offers a gentle yet technical (from end-user perspective) introduction to the Linux operating system, using Gentoo Linux as the example Linux distribution. It does not nor will it ever talk about the history of the Linux kernel or Linux distributions or dive into details that are less interesting for Linux users.

1. Linux sea available in html format.
2. Support forums: No

#21: Oreilly Commons

The oreilly publishing house has posted quite a few titles in wiki format for all. The purpose of this site is to provide content to communities that would like to create, reference, use, modify, update and revise material from O'Reilly or other sources. The site includes books about Ubuntu, Php, Spamassassin, Linux, and much more all for free.

1. Oreilly commons available in wiki format.
2. Support forums: No

#22: Ubuntu Pocket Guide

This book is written by Keir Thomas. This guide/book is a good read for everyday Ubuntu user. The purpose of this book is to introduce you to the Ubuntu operating system, and the philosophy that underpins it. You can download a pdf version from the official site or order a print version using Amazon.

1. Ubuntu pocket guide available in pdf and print formats.
2. Support forums: No

#23: Linux: Rute User's Tutorial and Exposition

This book covers GNU/LINUX system administration, for popular distributions like RedHat and Debian, as a tutorial for new users and a reference for advanced administrators. It aims to give concise, thorough explanations and practical examples of each aspect of a UNIX system. Anyone who wants a comprehensive text on (what is commercially called) LINUX need look no further--there is little that is not covered here.

1. Linux: Rute User's Tutorial and Exposition available in print and html formats.
2. Support forums: No

#24: Advanced Linux Programming

This book is intended for the programmer already familiar with the C programming language. It take a tutorial approach and teach the most important concepts and power features of the GNU/Linux system in application programs. If you're a developer already experienced with programming for the GNU/Linux system, are experienced with another UNIX-like system and are interested in developing GNU/Linux software, or want to make the transition for a non-UNIX environment and are already familiar with the general principles of writing good software, this book is for you. In addition, you will find that this book is equally applicable to C and C++ programming.

1. Advanced Linux programming available in print and pdf formats.
2. Support forums: No

#25: LPI 101 Course Notes

LPIC-1/2/3 levels are certification for Linux administrators. This site provides training manuals for LPI 101 and 102 exams. These are licenced under the GNU Free Documentation Licence (FDL). This course material is based on the objectives for the Linux Professionals Institute