From the obvious to the less obvious:
1. Adobe Flex Builder: Most Flash developers swear that this is the easiest way to develop rich Internet applications for the web and the desktop using Adobe Flex and Adobe AIR. Understandably the most extensive IDE for Flex available on the market. A free student license, a 60-day free trial for others followed by a license at US$249. This is the only cross-platform IDE in this list.
2. SapphireSteel Amethyst: A few days ago, the Beta 3 version of Amethyst was released. An edition of Microsoft Visual Studio 2008 is a prerequisite, on which this can be installed as a plugin. One could also use this on top of SapphireSteel’s Ruby In Steel. Good documentation and tutorials are available on the product webpage.
3. Ensemble Tofino: Tofino is similar to Amethyst in that it is an Adobe Flex plugin for Microsoft Visual Studio. Free for download, like Amethyst.
4. FlashDevelop: I’ve discovered this only today and I’m very glad. FlashDevelop is a free open source standalone IDE though it hasn’t got a design view and all that. It is being developed primarily by Mika Palmu of Finland and Philippe Elsass of France and is hosted on Google Code repository and distributed through the MIT license. Apart from the usual code editing features, it has got features for code completion, code generation, code documentation as well as SWF/SWC exploration. Take a complete features tour here. Try downloading and using it, it is just 3.7MB in size, and donate a dollar if you like it. I’ve just started trying it and I like what I’ve seen so far. Mika, Philippe, and folks, you are doing a great job.
What other IDEs (or plain text editors) do you use to develop Flex apps?