I've recently installed the new Eclipse IDE on my mac, using Aptana for my main PHP and web development. I've been so impressed in this product, that it's made me curious about Eclipse's other plugins available for more development.
I want to install Flex 3, without buying the FlexBuilder app, so back to Eclipse I go. Wesley Swanepoel, a top designer developer at a UK-based agency, (and also a personal friend of mine) has put the following help on his blog. I find his blog really rewarding - letting him do all the pioneering and then putting his experience online to assist others (newby's to Flex and Flash, like me)!
Check it out if you're getting into Flex.