What Steam game have you the highest playing time?

406 hrs in Inside a Star-filled Sky, and the amount of playtime is increasing steadily. Not sure if that counts though, as I'm not actually playing, but just letting it run in the background.

CS:GO 249 amazing hours

Plants vs. Zombies 42h

1. Don't Starve 30 hours
2. GTA IV 30 hours
3. Sleeping Dogs 26 hours
4. Rust 16 hours
5. Mafia II 13 hours

Path of Exile. Great game and FREE!
I always want to make more characters.

And apparently cod:mw2... game sucked but i enjoyed playing it sometimes.

900 hours of Killing Floor

Highest legit? Garry's Mod.

Sadly, Team Fortress 2 (259 hours) :(

Sanctum 2. However i think L4D should be there, I was playing it on Hamachi way before I bought it when they put it for sale 2 xmas ago.

Skyrim - 507 hours.

Supreme Commander 2: 800+ hours

Arma 3 however i tend to play mostly offline so most of my games don't show the gaming time :-(

EVE Online 428 hours

2392 hours on Borderlands 2.

TF2 has my most hours played because of idling and actual playing

Warframe since I mostly idled in Team Fortress 2 :p

Right now it's currently Don't Starve for me with 192 hours clocked on Steam (irl it's more like 292 hours, since Steam doesn't calculate offline play in games for some reason... >_>), but FTL: Faster Than Light is gaining ground since the release of the Advanced free DLC, I have 100 hours clocked on Steam but just like Don't Starve it's more like 175 hours since Steam hasn't added my offline hours to the total.

1742 hours TF2 (just a few hours are idling, below 50h)
1714 hours Dota
250 hours CSGO (100ish are probably idling)
190 hours Terraria

These are my top ten, based on Steam Gauge:

1. Just Cause 2, with 285 hours.
2. Burnout Paradise, with 87 hours.
3. Sims 3, with 85 hours. (Doesn't really count since I lent my computer to someone for a week and they played Sims 3 all day, heheh.)
4. Saints Row: The Third with 81 hours.
5. Sleeping Dogs, with 73 hours.
6. The Elder Scrolls V: Skyrim, with 69 hours.
7. Far Cry 3, with 39 hours.
8. Mafia II, with 30 hours.
9. Killing Floor, with 30 hours.
10. Just Cause 2: Multiplayer Mod, with 30 hours.

cod MW3 404 h

Euro Truck Simulator - 100 hours. Yes, I'm really sad person..

Doto with only 1534 hours.

Europa Universalis IV

Apart from idling in TF2 (obvious #1):

1. Audiosurf = 104 hours and counting;
2. Just Cause 2 = 92 hours and counting;
3. The Binding of Isaace = 64 hours;
4. Deus Ex: Human Revolution = 64 hours and counting;
5. Plants Vs. Zombies = 63 hours and counting;
6. Saints Row: The Third = 61 hours and counting;
7. Mortal Kombat Komplete Edition = 60 hours and counting;
8. Divine Divinity = 58 hours;
9. Left 4 Dead = 48 hours and counting;
10. BIT.TRIP.RUNNER = 48 hours and counting;

Excluding TF2 because i made mostly was idling.

1. A.V.A - Alliance of Valiant Arms (1,182 hrs)
2. Counter-Strike: Global Offensive (267 hrs)
3. Torchlight II (79 hrs)
4. Counter-Strike: Source (60 hrs)
5. DayZ (56 hrs)
6. Call of Duty: Black Ops II - Multiplayer (40 hrs)
7. Saints Row: The Third (36 hrs)
8. Rising Storm/Red Orchestra 2 Multiplayer (27 hrs)
9. Left 4 Dead 2 (26 hrs)
10. Dead Island (23 hrs)

And that's it.