I\'ve never developed Flash before but I have a project where I want to use an ActionScript 3 library and I\'m not sure what tools I need to start. To further complicate thi
A Windows or Mac box. :-)
just kidding, actually you can use the Flex 3 SDK mentioned above and compile using the CLI. If these AS3 Libraries are indeed Flex libraries (sometimes there are discrepancies between what works in Flex, what works in CS3, and what works in both) then you should be able to import them and use them.
Also, I've never tried it myself, but Flex Builder is based on Eclipse and I believe there's a way to configure Eclipse to compile with the opensource Flex compile if you'd rather work in a graphical IDE.
I have written a blog entry on how to setup Eclipse + AXDT + Flex SDK to prepare your mxml/actionscript (as3) development environment on Linux. If you are interested, you can have a look here: http://netpatia.blogspot.com/2009/09/flash-development-on-linux.html
While FlexBuilder is good, it lacks many of the features that Powerflasher includes in FDT (Flash Developer Tools) for Eclipse. FlexBuilder is missing things like Templates, better formatting, auto-fixing of warnings and errors. It runs on Eclipse making it cross platform, but be warned that they do not test in a linux environment (however people have had no significant complaints).
See all the features here: http://solutions.powerflasher.com/index.php?id=136#feature_03
You could try Haxe. It isn't really ActionScript but it is very similar (and you could also compile to the server side or JavaScript).
I've had very good experiences with Sprouts. It is a ruby wrapper for the Flex SDK that allows building with Rake and easily hooks into continuous integration engines.
http://www.projectsprouts.com/