FlashDevelop is a stand-alone IDE specifically designed for ActionScript (though the version 3 supports AS2, AS3 and HaXe) that is written in .NET and is easily expanded with .NET plugins. As it is .NET, it is currently only available for Windows, but with Microsoft starting to show support for Mono, it may well be appearing in OS X and Linux in the not too distant future.
! Warning: this post hasn't been updated in over three years and so may contain out of date information.If you are a Flash designer, the chances are you are happy with The Flash Pro development environment. It has been created with designers in mind and meets your needs well. If however you are a Flash developer, then you likely find Flash Pro truly awful as a development tool. If you fit the latter category, you may not be aware of alternatives to Flash Pro. There are a range of them including SE|PY, SharpStyle Neutron (a VS2005 addon), Power Flasher (an Eclipse plugin) and ASDT (another Eclipse plugin). However by far the best of the bunch (in my opinion at least) is FlashDevelop.