Which Steam game have you got the most play time on?

Killing floor, about 600 hours

TF2

TF2 with 55.5 hours.

TF2 a modest 39 hours

Going with the general consensus: TF2 with around 750 hours :-)

BFBC2

TF2 with 180 hours. Second place is Terraria with 95.

Tf2, 850+ hours, no idling whats so ever.

I know someone with more then 4000 hours in Gmod.

Bad Company 2!!!

Left 4 Dead - ~273 hours. I swear on my mothers grave (although she isn't dead yet...) it said 273 a month or so ago. Now it says 238. Weird.

Counter-Strike - 767h

Counter-Strike - 1754h

TF2 for about 180 hours

2749 hours of TF2. Mostly idling.
Second place - Black Ops with 469 hours.
Third - Killing Floor with 243 hours

RIFT. 150 hours

Tf2 :O A LOT! :O

Team Fortress 2 with 2360 hours.

98-L4D2
61-BC2
37-CSS

Call of Duty Modern Warfare 2 around 1400 hours.

killing floor. - 658 hours. ;3

tf2, 1200h+

58 hours on terraria

Portal 2 39.6
Borderlands 46.9
Garry's Mod 89.5
Team Fortress 2 365.5

Modern warfare 2,With almost 1400 hours