Right now there aren’t any good open-source development environments for Flex. Everybody seems to be using Flex Builder, which you have to pay for. For me, I like things free especially since my primary web development platform is Ubuntu 9.10. I did find AXDT which, which I feel shows the greatest promise in evolving as a free alternative to Flex Builder. Plus it helps that the primary target seems to be Ubuntu.
For more information, you can go to: www.axdt.org. I have got it working with the latest Flex 3.5. I am having some trouble with Flex 4 though but that’s still in Beta. Primarily I wanted something to allow me to edit mxml and Actionscript files not caring much about layouts. I have also been able to get it working with AMFPHP. So far it has done most of what I want.