Jazzpunk is fantastic game but completely insane.
I have been really enjoying Gas Guzzlers Extreme lately. A racer with Serious Sam type weapons, arcade handling and it looks great.
The S.T.A.L.K.E.R. series is great and very atmospheric semi apocalyptic RPG but it is also difficult sometimes very.
I cannot recommend buying it but try the demo for Euro Truck Simulator 2. I bought it and have over a hundred hours in it but it is not for everyone so try the demo available on SCS's website first.
Loadout is a fast cartooney and really fun FPS not unlike TF2 but I find it more colourful and fun.
There are also.mods for games on consoles that vastly change them to something that could never exist or provide game play not available on console.
EDIT: Looked at my library got some more.
Audiosurf is a fun rhythm action game, you use your own music and it makes a track to play on.
Bastion is just beautiful hack and slash/rpg with great music.
Defense Grid is a fun but difficult tower defense,
Another tower defense, Dungeon Defenders, great with friends.
Floating Point gets honorable mention, it is free and relaxing.
Minecraft, while available on consoles, is a totally different beast on PC, light years ahead of the console releases.
Race The Sun is like a infinite runner type game but from a third person perspective and really addictive, super simple styling and game play.
TrackMania is a quirky racing game with a full track editor.
Uplink, a hacker simulator of sorts, really good fun but unrealistic.
World Of Goo is always going to be fun, sort of building, physics, puzzle type game.