- FlashDevelop. Up there with FDT and FlexBuilder as one of the best ways to write ActionScript. I use it together with the Flash IDE (using the IDE for layout, animation and compiling), but it also works with the official free Flex SDK and with unofficial free alternatives MTASC, swfmill and haXe, so you can create Flash games without spending any money at all. If you've ever used Microsoft's excellent Visual Studio, this is the closest thing for Flash developers. FlashDevelop knows the contents of all your classes and provides amazing code completion that seems to know what you want to do before you know yourself. This also gives another good reason to use static typing if you're not already.
- Paint.NET. Lots of people know that Gimp is a popular free alternative to PhotoShop, but I much prefer the simplicity and ease of use of Paint.NET - it's easily as good as PhotoShop for doing simple tasks like cropping an image. UPDATE! For a open source tool for creating vector graphics, check out Inkscape - it looks pretty handy if you don't have Illustrator or the Flash IDE. It exports SVG, which I believe you can include directly in Flex SDK projects.
- Audacity - for recording and editing your game's sounds. UPDATE! To create wicked retro sound effects check out sfxr!
- Blender - a powerful free alternative to 3D Studio Max and Maya. Plug-ins are available to export directly to PaperVision3D and Away3d - sweet!
- Subversion, Tortoise SVN and WinMerge - invaluable tools for managing your source code. As a recent convert, I highly recommend you do this!
- Easy PHP and Red 5 free server-side environments for developing high score boards and multi-user games, etc.
- Eclipse or NetBeans - primarily Java development IDEs (useful when developing SmartFox Server extensions), they also support a range of other languages, such as PHP. The notepad days are truly over - get an IDE!
- FileZilla - you'll want to upload your game at some point - I recommend using this!
- Firefox. Firefox is a great place to test Flash games, thanks to useful plug-ins like FlashTracer, LiveHTTPHeaders and Tamper Data.
- Open Office - use the word processor to spell check your instructions screen, and the spreadsheet application to count your piles of money!
Creative Commons image by KarraMarro
That's your lot! More open-source Flash projects can always be found at OS Flash.