FlashDevelop is a great light-weight tool for developing in ActionScript, it makes joint development easy and is very flexible for different project types.

We all know that Flash has been modified as a danger due to all it's security issues, but you can't argue as a software developer on how elegant ActionScript 3 is, at least that's my opinion. The fact that you can get a free IDE to work in Flash, that is compatible with Adobe Flash and Adobe FLEX projects, that it has it's own package manager to download debug tools, compilers, additional packages and more is just awesome. It's free, it's constantly maintained and updated, has great support, tons of documentation and you can keep working in ActionScript 3, no matter what happened with Flash and with the new HTML5 technologies.

I like it because It is a very lightweight IDE, it can work in diferent PC without incovenient. You can create several kinds of project like AS3, Adobe AIR, Haxe and even PHP. I love its shortcuts they are life saver.