So the only game you seem to hear about these days is Quake. Well, it ought to be; the graphics are superb, the gameplay kinetic, and the hunt for secrets, strategies, and something to kill just plain addictive. The full version is finally hitting retail (and online), but if you haven't had at it yet, you may want to try out Dimension of the Damned, the shareware first episode.
With just seven small levels, it's pretty short, but each time you play you're likely to find something new.