Now this is my kind of question!
I'm a brush-junkie...
One thing I'll mention, before the links, is look at the copyright that surrounds the brushes and respect it. There are TONS of great royalty, monitary, credit
and copyright free brushes out there, but if something says to credit the creator, do, it's only fair. Some will just be linkware and the like but anything like that must be respected if you want to be respected yourself.
On to the links...
One of the best resources is DeviantArt
...here... if you go to browse-application resources-photoshop brushes, you'll get tons and then you can sub-search within that if you're looking for something in perticular.
Another good but limited site is
this one
really DA will have nearly everything you might want but it can be a trawl... I've been on there long enough to only really need to look at the latest releases but the back catalogue might take you a while. There are tons of clubs on there too just for people who make brushes to show off their work.
And if it's not enough there are sites where you can buy cd's of brushes and use them as you will... sure you're paying but after that they're yours.
Any more questions please feel free to ask.