Are there any open source (read free) alternatives to Flex Builder? I do not qualify for their student waiver (sad)
There are some good options here.
http://www.riaspace.net/2009/01/flex-developer-tools-ides/
FlashDevelop seems to be the most popular.
The answers to these questions should help you:
Flex development on Linux, what’s a good free environment?
What is the best (most productive) FLEX IDE?
Since the flex SDK is free and open source you can really used any IDE you already use, only do command line compile.
If your on linux the flex builder plugin is still free, and from the looks of things it will stay free for a while as I don't see them putting more work into it anytime soon. I personally think its perfect as I don't use design view ever anyway and that is most of what is missing you still get great code hinting and so forth.
Whilst FlashDevelop is windows-only, there's also AXDT: open-source cross-platform alternative for Flash development.
It's not a competitor to Flash Builder or FDT that are way more comprehensive, but it makes a good setup for small things or for those just starting to develop for Flash Platform.
They also have an unemployed waiver now too:
http://www.adobe.com/devnet/flex/free/index.html
Overall it seems like they're saying "try me out, if you like me, upgrade me to premium" especially since the premium version offers a boatload of new tools.