I have to say Quake Wars, unfortunately. I had been waiting for the game for awhile, but I have no inclination to pick it up after playing the MP demo. It feels like BF2 with Quake weapons and controls (and I do like the weapons, it has the very tight Quake feel for combat). Vehicles are very fragile and IMO handle badly, though. The demo map was also balanced very badly in favor of the Strogg. I like the idea of ongoing ranks, etc., but the demo didn't really support it. The gameplay also felt too slow-paced in comparison to, say, UT3. It would entice me to buy an 8800 GT bundled with it, but I won't be picking it up for more than $20 alone.
Edit: Oh, the other games! I like the UT3 demo, although it doesn't run super well on my machine (need to get that 8800 GT I spoke of). The Crysis demo is fun as well, although it runs like dog shit. HL2 Ep2/Portal are fun, although IMO not quite worth the price - too short.