The best (and frankly, only) render site I've seen is Game Renders. But if you go there, be sure to look through the forum rather than just the gallery, because there is only a small portion of the selection in the gallery. If anyone else has any good render sites, post them here.
Thanks Sirron for giving me the info about going through the forums.
Well, the render is an image like the combine in your sig, or the Spartans in some of RH's sigs. It's just an image that you put into your sig, but it usually needs to be cut out from a larger picture to make it transparent (quite tedious work, although I enjoy doing it ).