• MyDB Studio - Makes a useful administration app for MySQL and also has some design capabilities such as the SELECT wizard to help with overly complicated queries.
• SQLYog - Provides an excellent GUI for designing and administrating you MySQL database.
• WWW SQL Designer - A really funky Ajax database designer.  It has many features normally only available in offline apps.  It’s got a lot of promise although its still being very actively developed it’s stable and fast.
• OpenOffice.org Base - I know its certainly an unusual one to incude in the list but OpenOffice Base does have good support for working on a MySQL system using its Java interface.  Worth a look but if you have a huge database then it can take a performance knock.
• AutoMySQLBackup - Why go to the special effort of producing scripts to backup when you can use this.
• PHPMyAdmin - Been around for years and still going strong.  Its not the most speedy when handling large sets of data (try offline apps such as SQLYog or MyDB) but its got every functionality you can shake a stick at.
  

Security Tools

• SQLIer - A SQL injection tool which you provide a URL and it does all it can to perform SQL injection.
• SQLMap - A blind SQL injection tool.  It provides support for PostgreSQL, MSSQL as well as MySQL.
• Absinthe - An application which is available on Linux and Windows.  It provides MySQL injection brute forcing on your server.
• SQID - SQL Injection Digger was created in Ruby as a command-line tool for brute force SQL injection testing.
  

Optimising MySQL

• MySQL Performace Blog - For regularly updated information on optimising your MySQL install.
• SQL Commandments - 25 rules of optimized database design.
• 10 Tips on Optimizing MySQL - From 20bits.
• Another 10 Tips - They just keep on trucking with those optimisation tips.

Knowledge

• MySQL Tutorials - Great collection of MySQL tutorials.
• MySQL Cheat Sheet - Provided by the prolific producer of cheat sheets over at I Love Jack Daniels.
• 10 Common Mistakes of MySQL Design - Learn what not to do!
• MySQL Clustering How-To - Exactly what it says.
• MySQL Master-to-Master Replication - If you need to setup replication in a ring then this is your place.
• MySQL vs. Postgre - Its good to compare.