F2P Games on Steam.

PATH OF EXILE - BEATS DIABLO 3 ETC :)
Fallen Earth - when i first played it 1-2 years ago or 2-3 ,people used to call it a Fallout MMO :D
Zombie Panic Source
Maybe Air Mech too
MMORPGs - Fallen Earth & Path Of Exile are tied for number 1 in my list as free mmorpgs on Steam :D ( heck even outside Steam) , Lord Of The Rings Online,Neverwinter Online
etc

Elsword

Planetside 2

TF2, awesome game loved it, after around 200 hrs it gets boring, though with custom maps, this game is EPIC

Spiral Knights, looks like a childs game, though is very fun and addicting; though it does get quite boring after a while

No more room in hell, COMPLETELY F2P, you cant pay anything in this game, a very realistic zombie game, very fun, though I like the arcade zombie game more like L4D2

Star Trek Online is a solid MMO. The game's been getting some much needed improvements and overhauls in the last year or two, and the combat & customization are a lot of fun.

Dungeonland looks really fun though I only recently rediscovered this one and my friends & I haven't gotten around to playing it yet so I can only tell you that watching footage of it has been thoroughly entertaining, especially when one of the players is the dungeonmaster and gets to spring all kinds of nasty surprises on his (former) friends.

Star Trek Online has been great & doesn't need a dime of subscription to play it. DC Universe & Champions Online good too if you want that genre.

There are some free fantasy MMOs on steam but haven't been too thrilled with the gameplay on those but you can look up Runes of Magic or just buy Guild Wars2 (free once you buy the game) where there's updates in GW2 world about every 2-3 weeks keeping it quite alive.

edit-forgot about Age of Conan on steam, also fun & free to play.

Warframe

Path of Exile is really fun to play. Neverwinter is also pretty good.

I quite like Cryptic's MMOs: Star Trek Online, Champions Online and Neverwinter. DC Universe Online is an interesting and fun game too; and if you want to try something classic, then there is Everquest (1 and 2), Lord of the Rings Online or Age of Conan. If you like the postapocalyptic setting, then try Fallen Earth.

i would recommend LOTRO, path of exile, APB Reloaded and Mighty quest for epic loot :)

Path Of Exile. Trust me, better then Diablo III

Planetside 2. Totally F2P, things that you can get only by cash are the cosmetic stuff. You can also buy gear like weapons/stuff for vechicles for cash for fast unlock, but you can get it with normal gameplay. Best things costs 1000 credits and after they added some misions, alerts and exp bonus to areas, earning credits is much faster. One week of active gameplay and you have 1k or even more.

I'll have to recommend Marvel Heroes, it's really good. Warframe and Path of Exile are also complete full games.

Team Fortress 2 is probably the best one. But I play Hawken now. It's quite fun.

Loadout and Team Fortress 2 - the only f2p games that aren't pay to win

Dota 2 had me pulled.Go gets dem.