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FlashDevelop comes without any cost.
Pro Good code completion
FlashDevelop's code completion is pretty good.
Pro Haxe development support
FlashDevelop has first-class support for Haxe development, the open source toolkit based on a modern programming language and cross-platform library.
Pro Excellent support for Actionscript 2 and 3 (Flash)
Although everyone claims Flash is dead, it's still quite useful for game developers due to its rapid compile and run times, as well as its great debugging functionality with FlashDevelop.
Pro Good number of project templates
While it's project template system is not the best compared to it's competitors, it still is decent and is a good way to generate some boilerplate code.
Pro XML/HTML completion
FlashDevelop has XML/HTML completion aside from code completion.
Pro .NET Framework 2.0 application
It's windows only, but has tremendous support from plugin developers and a dedicated team that's been developing it for close to 10 years.
Pro Source-control support (svn, git, mercurial)
Pro Great debugging
FlashDevelop provides very efficient debugging features.
Pro Supports Zen-coding for HTML
This is very useful for carrying out high-speed HTML coding and editing.
Pro SWF/SWC exploration
Pro Great project compilation
FlashDevelop facilitates project compilation.
Pro Decent code generation
Although the code generation can't really be called top-notch, it's decent and sufficient for most developers.
Con Windows only
FlashDevelop is for Windows only, and it's not cross-platform either.
Con Haxe debugging is in its infancy
Although FlashDevelop supports breakpoint debugging on Flashplayers, native Haxe applications (C++) can't be easily debugged within FlashDevelop.