MiniBuilder is a very lightweight Flash builder, created for developing programs written in ActionScript 3. It's written in ActionScript itself.
Extending, changing (and even integrating MiniBuilder with other software) is easy: MiniBuilder is itself written in your favourite language, AS3!
MiniBuilder will become a strong IDE, both online and desktop, and will promote using ActionScript in unexpected places like on server-side, native desktop applications, embedded devices, etc. Also, other ActionScript projects can use the code completion engine in their own solutions. More Info »
This and other of it's unique features and concepts will make it the IDE of choice for a lot of AS3 developers.
MiniBuilder will benefit from Adobe AIR's ability to launch applications from the browser. Code samples, online tutorials, code libraries will be able to provide a direct launch link for their projects: "launch this project in MiniBuilder" (This feature will be available in the first beta version.)