FDT by Powerflasher is a commercial alternative to Adobe's Flash Builder IDE. While they both work on similar technologies, ActionScript, MXML, the Flex SDK etc. they both have different orientations and feature sets offered.
FDT is geared towards core developers, and thus lacks some of the more powerful design features of Adobe's Flash Builder IDE, but offer a much more powerful coding workflow. FDT has the added advantage of being available for Linux, which Adobe tried with Flash Builder (which was at that point of time called Flex Builder) but eventually abandoned.
Powerflasher FDT 5, which will be releasing in October, will not only support the above-mentioned new features but will now also be available for free! The current three editions (Pure, Plus and Max) will be replaced with just two editions, Free, and Max. The Free edition, as is obvious by its name, will be available for free. The price of the Max edition will be reduced to 499€.
With a free edition of FDT available, the choices of free development environments for Flash developers are increasing; another popular Flash IDE being FlashDevelop, which is an open source project. Hopefully Adobe will follow suit and make a free version of Flash Builder available to all. The Flex SDK is already free and open source.
In the meantime you can download a trial of FDT 4, or check out FDT 5's overly-graphical HTML-based webpage here.