Skyrumz.

TF2

SMNC, Dota 2, Alien Swarm, APB

For non-steam games: World of Tanks, Tribes Ascend, Runescape, Tinker (GFWL), and my personal favorite Space Station 13 on Byond

I don't know what you guys are talking about but moonbase alpha is definitely the best f2p game there is, just look at it in what other game can you jump around the moon and fix wires like a true astronaut another plus is that playing this game will be the last time you will ever see NASA on the moon :O anyways what is better then slowly jumping around and riding moon rovers with your best friends.

Hat Fortress 2

tribes ascend

Dota 2.taaadaaaaaaaa !

Team Fortress 2 and Dota 2 :)

Team fortress 2 no further discussion :P

WELL, ALL PEOPLE HERE WHO SAY THAT TF2 IS THE BEST F2P GAME IN STEAM, THEY ALL GOT PREMIUM, SO THEY PAYED FOR IT, SO IT'S NOT FREE FOR THEM...

DC Universe Online.

Not on Steam anymore since going F2P, but Aion trumps all of these. :D

dota 2 is king of all paid and free games.... hope you enjoy it :)

Peggle Extreme

TF2 or Realm of the Mad God if you don't mind the retro graphics. Plus RotGM is a really small file so won't clog your HDD too much.

TF2, Dota2, Alien Swarm and RotMG

Dota 2 and TF2 is the best F2P games on steam... :3

Team Fortress 2 and Spiral Knights.

Rusty Hearts is the best F2P on steam

TF2 in my opinion sucks... it isn't fun at all, never liked it

I've tried out quite a few. CrimeCraft, APB Reloaded, TF2 (horrible game), Trackmania, Realm of The Mad God, DC Universe Online, Champions Online and I think maybe a few others. All in all, they were pretty bad. CrimeCraft, DC Universe, Champions Online and APB were all bad, duplicates of each other. However, APB is pretty fun with friends and isn't all that bad. Realm of The Mad God isn't my kind of game but I can see some people enjoying. Team Fortress 2 was horrible, I don't think I spent more than 10 minutes on it and I don't want to ever go back.
Trackmania however, was brilliant. Probably one of the best racing games I've played. Very creative servers, great maps, fun with friends, the racing is actually really good too.

tl;dr Get Trackmania. It's awesome.

Hat simulator 2

Dota2

TF2 of course if you're like an FPS gamer.

Realm of the Mad God