Thanks to the lingering after effects of Steam and other online sales I am vacillating back and forth between several games at present:
Left 4 Dead 2: All the goodness of Left 4 Dead 1 with a good deal extra on top. Considerably more challenging too in my opinion. I have completed three campaigns solo and have dabbled in a bit of online co-op on those campaigns. I don't like going multi player on campaigns until I have gone through them first in single player.
Bioshock 2 Multiplayer: With plasmid in one hand and machine gun in the other this is great fun. It has a level up mechanic like so many modern shooters but thankfully you only have to advance a couple of levels to get your hands on some decent weaponry. Servers are not terribly busy but I found it easy enough to get into a team deathmatch.
Overlord: Play as the Demon lord commanding an army of demons. I am only a couple of hours into this but it seems very imaginative and surprisingly pretty to look at.
Men of War: Just bought from Gamers gate where it is on sale for a few euros. I loved Company of Heroes and this has a similar feel but perhaps a little bit more complicated. Looking forward to it if I ever get time to finish the tutorial.
Chrome Specforce. It was only €0.99 in the Steam sale but it has not aged well . Brown depressing graphics and fairly standard shooter play. I did get a couple of hours entertainment out of it before moving on though.
The Birth of America: This is the type of game I feel I really should be playing - slow cerebral strategy gaming with lots of embedded history. I actually think I would enjoy it but to be brutally honest I may never get around to spending the time to get up to speed. I did play through the short tutorial though.
Plants versus Zombies. My wife is a Pop Cap fan-girl and I bought this for her in the steam sale and scored major brownie points. She absolutely loves it and has played it compulsively this week finishing the main campaign. Slight concern - now that my wife has her own Steam account she will be able to look me up and see just how much time I really spend on games (60 hours playing Steam games in the last 2 weeks! - that surprised even me.)