haXe

haXe is the follow-on for the Ming SWF library; instead of compiling Actionscript, they decided just to define their own language that could then compile to SWF, Javascript, and some other stuff.

I find that a tad irritating because I'd like to have something that could compile Actionscript 3.0 into SWF for me that was open-source, but the effort itself is pretty cool and worth looking at.