F2P Games on Steam.

some

Try Neverwinter, it was really quite good (except that the female voice for my character was terrible).

Marvel Heroes is decent. The big thing for it is it isn't Pay to Win.

They even have a decent selection of heroes for your first (only free) hero.

Every hero can also be unlocked via in-game drops. It will, however, take a while.

Realm of the Mad God.
WARNING: May be hazardously addicting to your health.

8-bitMMO is f2p i think

Path of Exile

Path of Exile or The mighty quest for epic loot

For some reason I actually kind of like F2P games more than games that cost money to play (which I own a lot of.)
But here's a list of some games that I think are good:

Team Fortress 2 (My favorite F2P game)
Tribes Ascend (not that many people play it now though)
Vindictus (I guess not that many people play it now?)
BlackLight: Retribution (Fun game, F2P kind of limits what you can do though)
Airmech (I haven't played much of it, but it feels like a promising F2P game title. I'm not too sure about how much of it is Pay 2 win though.)
No More Room in Hell (A really great zombie game, the entire game is F2P. So it's completely a free game. And a good one at that.)

A lot of people like Warframe, I don't think it's that great. And the game is pay-to-win.
Spiral Knights (The game is casual and the overall feeling is kind of meh for me. But the music and the artstyle and the overall calming peaceful feeling is just one that can't be beat. It's a still a k, game though. I just don't it's THAT great is all.
Path of Exile (I've been meaning to try that out, but a lot of players are playing in and enjoying it. So maybe you'll enjoy it. Plus you get a lot of community involvement.)
Rift (I have been thinking about trying that, I've heard that it's good. So maybe you'll like it.)

Wow, thanks for all answers :) I've played the most of those games, but now i wanted to ask about last title - "The mighty quest for epic loot" It is good?

Path of Exile is pretty cool and I enjoyed Marvel Heroes quite alot.

neverwinter

Planetside 2, Hawken, Warframe, Dota 2, Team Fortress 2, Blacklight Retribution, Spiral Knights, Realm of Mad God

Marvel Heroes if you like Marvel and Diablo style..

Simply, DOTA 2

Spiral Knights

no more room in hell is great

TF2, Realm of the Mad God and No more room in hell are the best IMO. Planetside 2 is great but you need brand new CPU and 16gb RAM to play it because of huge map and thousands of players. Played War Thunder and Path of Exile too but don't really like it because of grinding.

Spiral Knights(I just love it)
City of Steam(started playing this game and it is fun, it's great, I think, don't usually play point and click MMOs :P)
Fallen Earth(it's really weird at start but it's fun)
Airmech(The only MOBA and RTS I can have fun with)
Everquest 2(I will start play this next weekend, but what I've seen so far it's fun and it has a great community)

Loadout is great.You would love it

Warframe and Blacklight R

I'd say Spiral Knights and Marvel Heroes. Neither feels really pay to win.

DCS is the best air combat simulation existing, however the free version only has one ground attack plane. For more stuff you gotta buy addons and not all of them are on steam.

Path of Exile and Loadout