FlashDevelop is an Open Source Script Editor for Actionscript 2 /Actionscript 3. This Open Source Script Editor is written using .Net and designed mostly for Actionscript 2 and Actionscript 3 development.

FlashDevelop can be used as an external editor for the Flash IDE or as a complete open source development environment.

FlashDevelop is a very nice option for those looking for a quality/free ActionScript editor.  Also, if you’re still writing in ActionScript 2.0 and looking to convert over to ActionScript 3.0, this is a great tool that’ll get you used to constructing and using classes.

FlashDevelop Features

• Nice docking interface
• Create classes from templates, embed assets
• Built-in support for many workflows
• Flash CS3, Flex SDK, Mtasc/haXe+Swfmill are integrated
• Outline panel to give you an overview of your code
• Tasks panel to help you track your project state
• Comment/uncomment, transpose/duplicate lines, change casing, etc…

Website: http://www.flashdevelop.org