FlashDevelop is a highly customizable lightweight script editor with extended support for class based languages. Although the program is designed mostly for ActionScript, other languages can be implemented quite easily too.
FlashDevelop is also very quick and simple to setup and easy-to-use as an external editor for the Flash IDE or as a complete open source development environment.
Project management with seamless SWFMILL and MTASC integration (using a portable standalone command line tool)
Advanced Actionscript completion & exploration with automatic classpath detection (even without project)
MTASC code checking with clickable error results
Integrated SWF player and socket logger
Automatically cleans ASO files of modified classes (new)
Files explorer (can create Flash 8 Trust Files for you)
Automatic JavaDoc creation from methods
Smart Actionscript help websearch on F1
Jump to class/member declaration on F4
Test movie in Flash 8 / MX 2004
XML, HTML/PHP, JS, CSS code highlighting,
Multibyte character encoding
Program menu customization with XML files
as2api GUI for documentation generation