A first person shooter where you can choose a class and fight and earn weapons for that class.

A good first person shooter where you are in the star wars universe during the clone wars, you can command a sqaud and give orders to them.

An awesome mod for Half life 2 where you can co-op Half life 2 or you can play together on the synergy maps. Also has its own workshop.

A 2D space game, a game where you have a spaceship and a crew. You have to make decisions between life or death.

The second Portal, includes co-op and community made test chambers.

The original Portal is a First Person puzzle game, where you solve puzzles with portals while an artificial intelligence is trying to kill you.

The first game VaLVE has made, this is what brought more games and the SDK.

A good First person Sci-Fi, probably one of the most popular games on steam aside from portal I don't see why you wouldn't have it.

A very fun First-person shooter, it is still in development and has a few bugs but it is growing and improving.