For Brusheezy, check out its sister sites linked at the footer as well. In this context, though, I guess only
http://www.vecteezy.com/ would be a point of interest. There's also
http://www.bestpsdfreebies.com/ though it doesn't seem to have as many free resources. It's not hard to find other such sites by just Googling around, anyway.
There are also tutorial resources, of course, though more often than not they're aimed at (paying) professionals (except for those on community sites like DeviantArt). The only one I can remember off the top of my head now is
http://www.photoshopessentials.com/ though Adobe itself also has
tutorials and even
videos. It's probably easier to just Google what you want to do (e.g. "how to make <name of effect> in Photoshop") rather than looking at random tutorials, though.
Obviously, most such resources are intended for Photoshop. I realise GIMP users are often marginalised, but it can't be helped. On the positive side, if there's something GNU/Linux users do better than others, it's their community effort, as reflected in the very own
plugin repository for GIMP. It also has a lot of things that can help you use Photoshop resources with GIMP.