There is an ongoing argument on the web right now about the "openness" of the iPhone OS and Adobe Flash platforms. Adobe's argument is that anyone can build a Flash application without having to worry about being allowed to distribute it.
This made me wonder… is it possible to build and distribute an AIR application without giving any money to Adobe (without violating any licenses, of course)? Are any of these tools free? Are any open-source?
Yes. If you are on a pc download flashdevelop(which is free) or if you're on a mac you'll need to buy either flashbuilder($699), fdt($129-$699), textmate($60 i think), or download minibuilder for free... then go over to adobe labs and download the latest flex sdk. Once you've gotten this far check out any one of the numerous tutorials out on the interwebs that explain how to create and compile regular as3 apps, air apps, etc...
I really like flashdevelop as it comes with project templates for pure as3 apps, air apps, etc. It makes life very easy. I can't speak to the other IDE's as I haven't used them, but I know some peeps that use FDT and they speak very highly of it (but it's not free). Good luck!
The AIR SDK and Flex SDK are both free and open source. They are the primary tools for building apps on the Flash Platform. There are free and commercial IDEs built on top of these SDKs.
Yes, you can do it all with free and open source software -- I'm a Linux user and a Flash/Flex developer, and am living proof! ;-)
I write all of my AS3 and MXML code in Vim, and compile with mxmlc or amxmlc. There are some syntax definitions and omnicomplete plugins for Actionscript that work pretty well -- not full autocomplete like you'll find in many graphical IDEs, but enough to make Vim a very good option for Flash and Flex development.
For a graphical interface, you can create MXML layouts with the designview AIR app, it's a basic alternative to the design view in Flex Builder.
I also have a short wiki article you might find useful about the command line tools for creating AIR apps here.
If you're on a Windows platform, FlashDevelop is free and opensource, and very good. I uses .NET 2.0 though, so doesn't play well with other OSes.