FlashDevelop comes without any cost.

FlashDevelop has first-class support for Haxe development, the open source toolkit based on a modern programming language and cross-platform library.

FlashDevelop's code completion is pretty good.

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.

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.

FlashDevelop has XML/HTML completion aside from code completion.

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.

FlashDevelop provides very efficient debugging features.

This is very useful for carrying out high-speed HTML coding and editing.

FlashDevelop facilitates project compilation.

Although the code generation can't really be called top-notch, it's decent and sufficient for most developers.