FlashDevelop is a popular, free and open source IDE for ActionScript and Haxe.
FlashDevelop is a free and open source (MIT license) source code editor for Windows.
FlashDevelop offers first class support for Flash ActionScript (2 and 3) and Haxe development: great and fast code completion & code generation, projects compilation & debugging, plenty of project templates, SWF/SWC exploration, etc. See: features & screenshots.
FlashDevelop is more generally a good web developer IDE with integrated source-control support (svn, git, mercurial), tasks/todo, snippets, XML/HTML completion and built-in zen-coding for HTML.