Metro 2033

Metro 2033 is one of those games that feels like more than the sum of its parts.
Really, it's just a fairly basic FPS - the engine is competent but not outstanding, the graphics are decent but nothing particularly special and the script / voice-acting is fairly average (the NPC barks are extremely repetitive and annoying)
But... somehow it all works and is really enjoyable. Perhaps it's the fairly unique setting, the unfamiliar weapons and the creepy atmosphere, but somehow Metro sucked me in in a way that not many FPS style games do. It's not BioShock or Half-Life 2, but it's WAY ahead of CoD and all the various clones/competitors.
Particularly the first half of the game, which involves a lot of creeping around in the dark, was extremely tense and enjoyable - it went down hill a bit for me in the latter part of the game where the weapons power up a bit and all of the ammunition you've been saving starts to blaze, but it was still fun.
I also liked the ending, and bonus points for letting me reload and see the alternative ending without resorting to YouTube!

I got this as part of the Humble THQ Bundle (and another copy free from a Facebook give-away); in terms of marketing these deals served their purpose brilliantly (not that it helped THQ) as I'll certainly be looking out for the sequel if/when it's released...