FlashDevelop is an open source project, created for AS2, AS3 and HaXe developers to make their lives easier. It’s an IDE which can be a good alternative for a Flex Builder from adobe. The main advantage is that it’s free! It’s also a lot lighter and you can use it to builde ActionScript 2 projects, ActionScript 3 projects (Flex 2 and 3, AIR) and even HaXe projects! It has a code autocompleter, panels for presenting errors, warnings and tasks. It’s very like Flex Builder, but it doesn’t have visual editor - but I think it’s not a problem for more advantage users who uses only code editor. Also if you need to find a help, you will have to use adobe on-line livedocs. If you are thinking about buying FlexBuilder, you should try FlashDevelop in the first place. It’s a beta version and it’s still far far away behind FB, but in most cases it has everything you need!
For persons who are interested in PureMVC, here is a short tutorial how use it with FlashDevelop.