CS, Minecraft, GTA, Portal

TF2. Yes I said it. Sorry.

Tribes Ascend, Alien Swarm, Cave Story, Stealth Bastard, GTA1/2 (free on rockstars site)

+1. My favourite, the game will not be deleted from my hard drive. Never!

TU MADRE

Stealth Bastard, Blacklight: Retribution, Spin Tires, Quake: Live.

Enemy Territory. Period.

Zombie Panic Source

League of Legends !

Scorched Earth 3D.

and Hedgewars.

I think its about time to post ENVIROBEAR 2000!!!!! again.

Team fortress 2 League Of Legends and yes I will say team fortress 2 regardless because you said best game ever that's f2p.

League of legends, then Team fortress

TF2 and that other game about stragety

The SCP Games.

Actually I'm liking a lot Command & Conquer: Tiberium Alliances, looks a lot like a Facebook Game, but it's quite well implemented and it has a nice learning curve.
The New Sim City game on FB will rock the genre, making that sh!t Cityville look like crap.

Me. Oh and I'm cross platform.

Spiral Knights!

Définitivement ACR.

Path Of Exile is pretty amazing too. Also big up to Blacklight Retribution !

Has no one tried Vindictus? It's on steam now so I would recommend trying it out. At first it's extremely repetitive, but once you hit like level 25-30, it gets way better.

Happy Wheels on the browser or Toribash and Sumotori Dreams are just plain fun, and mostly with a friend.

Tribes: Ascend and Dwarf Fortress. Definitely not TF2 though.

Action Quake 2, except it's a mod, not a full game...

Mmorpg : Aion ( somewhat age of conan and forsaken world )
FPS: TF2, AvA
Strategy: Age of Empires online

Quake Live.