Well, Flex is not exactly Flash. If you're looking to do true flash development using open source tools, check out the MTASC compiler.

And for a specialized (free) ActionScript IDE I suggest SE|PY. It's integrated with MTASC, has better syntax highlighting than the eclipse plug-in, and is my preferred tool of choice. (Even over Flash's built-in editor -- which does a mind-blowingly bad job at managing screen real estate.)

Agreed, MTASC has been a huge time saver for me over the last few years, I can't recommend it enough for AS2 development.

Another free AS IDE: http://www.flashdevelop.org

I've not used it a great deal (I use FDT with Eclipse), but I thought it was worth mentioning as it does support AS3 too. Windows only though.