I was looking to try out some Flex stuff at home, but of course I didn't have a personal Flex Builder license I could use. (A coworker of mine wisely advised me not to try using my work license at home.) So I looked around to see if there were any free (as in beer and as in speech) alternatives.
The one I found was FlashDevelop. Although the IDE looks Eclipse-like, it's actually built using .Net. While it's not yet 100% polished, my impression from about an hour of working with it is that it's reasonably mature and quite fast (speed is not exactly something you could claim was a strength of Adobe Flex Builder.)
It seems that it supports AS2 and Adobe AIR as well, but I wanted to try it out using Flex first. I needed to download and install the Flex SDK separately -- it builds your MXML files by invoking the Flex Compiler Shell. This means the "Build automatically" option isn't really practical here (although I'm not sure if Flex Builder does it differently) I probably need to download the Adobe AIR Runtime as well if I want to build AIR apps.
I'm not gonna demo anything Flex-y yet (I only tried it out for an hour!), but this means I can start working on a particular thing which I've been thinking about for a while. Hopefully I can announce it soon, though methinks it'll be months before I get anything sensible ready.