Someone recently drew my attention to this website: http://game-icons.net/

There are currently 942 icons, with more being added all the time. A large number of them were copied (with permission) from another collection that I mentioned a while ago, but this collection is growing and is much easier to search through (you can view by category).

I'm considering using them to create a graphical skill tree.

Oh nice, thanks for the link. There's some pretty nice ones there. :D

I work with vector graphics at my job (graphic artist for a sign/shirt shop), this might be a cool resource, and something I could probably contribute to.