STALKER: Shadow of Chernobyl. FPS with slight RPG tendencies occasionally. Sits proudly among my "Best Games in the Universe." You might not like it, though. It's pretty damn scary at some parts. Like, permanent brain-damage scary. At least I thought it was.
Deus Ex. Yet another which sits proudly among the "BGitU." FPS with strong RPG tendencies.
If you like the dungeon crawling aspects of RPGs, play <a href=http://members.cox.net/pmaranci/ROGUE.EXE>ROGUE. Yet another among the "BGitU."
Of course, the Half-Life games. Also among the "BGitU."
There's, of course, Fallout. Amazing game. Arguably the best dedicated PC RPG to ever be developed. Unfortunately, though, it does not hold such high regard as the previous games (to me).