Definitely Deviantart. It does require a great bit of searching to find the great ones though. Well worth it. You also might want to check out the 22 Pixels Photoshop Flock.
IMHO, www.bluevertigo.com has the largest collection of sites that have the largest collection of brushes. and a WHOLE bunch of other stuff too... and then there's deviantart.com which is really good too in the sense that unlike bluevertigo, you get what you're looking for fast, instead of getting sidetracked by all the awesome websites that you have to discover
here ya go: http://www.gfxfever.com http://www.brusheezy.com http://photo-shop-brush.com http://psbrushes.net and a lot more Hope this will help you.Cheers