I don’t use customised folder or desktop icons much these days but this set, entitled Ink, is great, based on tribal tattoo stylings. If there were other designs as good as this in the world of lurid, gum-drop-shaped, drop-shadowed reflectiveness, I might be more inclined to customise my folders now and then. Jamie McCanless is the artist responsible and you can see these and other designs on his site, including some nice GLBT and Pride-themed works.
PS: these are Mac-only.