If you want good plugins/good collection then -
TAL plugins -
SIR1 - Great Free Impulse-Response Reverb
Smartelectronix - Group of plugin devs, lots of good stuff, too many to list
Voxengo - Various free VST/AU Plugins
Glitch - Very popular glitchy multi effect
I notice this just got a tuts+ post which is basically the same thing, feedback on it though would be although it's a good idea in ways it wasn't much help and i will explain why. Reading it just seemed like a big list of the same developers packs/collections of plugins in a huge line most of which i know already and im guessing others would. So i was thinking wouldn't it be best to just post a link to the developers site and then a brief overview of the plugins they offer rather than just loads from that same site.
Maybe focus would be better on sites/blogs for finding new plugins/news like popular ones like KVR audio, sonicstate etc as they have all of these and more with databases but maybe thats just my view though. Anyway those are some good ones if not already known