I'm not much of a concept artist, but thought I'd contribute something to this great site.

http://dynamic.gamespy.com/~polycoun...052.html?00193

This is our reference image thread over at Polycount, it has quite a few links on various subjects. Constantly updated as new links are found.
If you don't feel like browsing the whole thread through, one of the members has gathered most, if not all, of the links to his site.

http://www.planetquake.com/polycount...ef_links.shtml