I’ve heard a lot of complaints from people about Flash’s built-in ActionScript editor. There has been third party development going on for a while. One application that I’ve tried in the past was SEPY, but the Mac version was buggy and I never tried the PC version. Lee Brimelow swears by it though.
Another editor that’s been in the works for a few years now and has finally yielded a full-fledged version is FlashDevelop. FlashDevelop is PC only now and I’m not sure if they’re planning to develop a Mac version, but this is a pretty awesome editor. Very quick with a lot of code hinting.
Give it a spin!