Flashdevelop + FlexSDK (read my blog if you need help setting them up).....with those combined you get the perfect programming environment for actionscript, for free. Also using flexSDK gives you the latest updates which take a lot longer to show up on the FlashIDE. But keep one thing in mind, even with the latest improvements in flash, it still isn't very good at performance, so for big games, if you don't optimize your code properly, there will be no point in switching to flash from silverlight.
My Site :- http://blog.bluemagica.com
I need a spriter, and a story writer for arcade fighter and a side-view RPG. If you can help, please pm me!