ThunderBolt is a logger extension for Flash ActionScript 2 and 3 as well as Flex 2 and 3 applications based on the Firebug add-on for Firefox.

For more information about ThunderBolt AS2 and ThunderBolt AS3 check out the Wiki.

It’s open source and based on the Mozilla Public License 1.1.
Updates:

* 10/14/07: ThunderBolt AS3 version 1.0 has been released with new examples using Flex 2 and Flash CS 3 based on AS3. New features added (memory snapshot, stop logging flag) as well - Happy Logging

Features:

* ThunderBolt AS2:
* Detailed information (class, file, line number, time, frame)
* Log levels (info, warning, error, fatal)
* Interactive tree view for complex object structures
* Profiling (time spend during code execution)
* Collapsible grouped output
* Filter log by classes and packages
* Interactive console (inspect and modify object on runtime)

* ThunderBolt AS3:
* Log levels: INFO, WARN, ERROR, DEBUG (FATAL, ALL)
* Tree view for complex object structures such as class identifier and its properties
* Custom LogTarget based on Flex Logging API including filters
* memory snapshot
* stop logging flag
* SWC components for logging using Flex 2 or Flash CS3

Read more

http://code.google.com/p/flash-thunderbolt/