I just happened to notice that Steam offers detailed statistics about which games people play. For example, I can see that 265 people (Steam users) are currently playing World of Goo. I can also see that 942 is playing Plants vs. Zombies. 1,771 Torchlight. 7,008 Garry’s mod. 43,478 Counter-Strike (which is just insane if you think about this a bit). And then finally, 50,518 are playing Call of Duty: Modern Warfare 2 (Multiplayer)
In case you want to do some sort of market research, the Steam stats system might come handy.