Definitely have to pick up Vampire the Masquerade: Bloodlines. A truly fantastic game based on an early version of the Half-Life 2: Source engine. Brilliant writing, great story, just all-around very entertaining. Genuinely creepy in a few parts and freaking hilarious in others. Keep an eye out on Steam for this, it seems to go on sale for $10 every once in a while.
Couple caveats: Activision laid off half of Troika's workforce and forced them to rush the game out very easy so it was extremely buggy at release. To their credit, a few of the devs actually pushed out a patch on their own a few weeks later. Point being, you need to patch it to really enjoy it. Activision forced Troika to release the game with some of the sub-plots and extra content completely unfinished, and the community is still extremely active patching it up and trying to flesh the game out to more what it should have been.
Two things you'll need: download the latest community patch. You may need the official path, too, but I'm not sure.
There's also a widescreen patch you may want, as well.
Spend a little time getting it up to date, and you'll have an absolute blast with it. It's one of the few games I still feel obligated to go back and play every once in a while.
Edit: Here's the
Wikipedia entry. Lots of good info. Also, because you probably won't pay much attention to it the first time through, I highly recommend just going into the folders and listening to some of the .wav files for the late night call-in radio show. The ads are brilliant (the political ad especially), and at one point the game's villaiin (a super-evil vampire type) calls in to the show to tell everyone the world is going to end. The attention to detail in that game really is amazing.
But at least now I know which games will be a good waste of my time and won't spend time playing something silly 
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