One fun game to play is Terraria its a Cheap game that is $10 which did stop being updated but now the developer is starting to update it again so i recommend getting it since it is such a fun 2D game with some much Adventure and Items

Half Life franchise amd garrys mod .

Fallout series and Blacklight : Retribution which is surpisingly good as hell.

killingfloor, it's like l4D2 but better ;P with some awesome slowmotion effects

if yolu want to see a video of it then check out my channel to the left <<<<<<

said this in another thread already, but if you liked Super Meat Boy, you might as well like The Binding of Isaac (it's from the same guys.)

also Terraria is fun, but i think it doesn't last long, however it's really fun to play with friends XD

Saints Row The Third, chessy jokes, but fun game nonetheless.

I really liked L4D2.
I only have 4 games, though. ._.

Magicka is really fun, and funny there is a lot of comedy in it
(or at least i think so)

Half Life series, Black Mesa, the remake was pretty good too, perhaps F.E.A.R. 2. Had tons of fun while playing that.

You should liek totes buy Euro Truck Sim. huehuehue

Well, to answer your question, what kind of game would you like? Shooter? RPG? Puzzle? Horror?

Left 4 Dead 2 - Shooter - It's a pretty awesome game to play, even more awesome with you have 3 other friends, and it's an absolute bloodbath with you have a total of 8 dudes.
Terraria - Sandbox - Pretty much a 2D version of Minecraft with a sort of RPG aspect It's pretty simple and pretty additive.
Half Life 2 - Shooter - It's a little bit old-school (and by a little bit I mean a LOT), but it you just ignore that fact, it's pretty fun. It's essentially the root of all modern shooting games :O
Portal 2 - Puzzle - Portal 1 is fun too, but it's also pretty ghetto. It can get pretty frustrating, but it's totally worth it
The Assassin's Creed series - Adventure - Technically available on steam. Relatively pricey, if you had to get only one of the games, get #2 or Brotherhood. There's just something about playing a handsome italian man that makes this game so amazing... Or maybe it's jumping out of the skies and killing your prey in silence. Probably my personal favourite game series of all time.
Borderlands 1 or 2 - Shooter/RPG - Oh my god, this game... Just... SO MUCH FUN. It's a shooter that has RPG elements like quests and item drops that has a co-op of 4 people. It's fun alone as well though if that's what you're into. It's an absolutely phenomenal game.

If you had to get only one of the games I mentioned above, it's going to be a close tie between Borderlands or Portal 2.

A very short but hilarious game is 'DLC Quest', I think I paid 2 dollars for it but its rather funny if you're sick of all the DLC things coming out for games that should have been included in the first place. Symphony is another really good game, its a music base top view shooting game. Enemies come in based on how the music is and there are random bosses that will come in that you have to defeat, you use the music on your computer but you can also use pre-loaded music I believe. I only used music on my computer for that so I'm not fully sure about the pre-loaded part.

Well, I have a shit load of games on my Steam ...

But i think my favorites are:
- The Penny Arcade adventures
- Castle Crashers
- Dungeon Defenders
...

and recently, Zeno Clash!

Bastion is a really nice game if you appreciate a good story and good music.

I'm a fan of puzzle games more than anything else...so my favourites are:

Puddle (amazingly addictive)
Snapshot, it's original and I love it
The Bridge, can be really confusing but It's a brilliant game if you like optical illusions.

If you like free steam games like me you should rlly try "Alien Swarm" store.steampowered.com/app/630/

I'm pretty sure they have Hotline Miami on Steam. Bash em up good

I personally love the civilization series. Civilization 4 is a great place to start and Civ 5 is the one with most fun along with the enhancement mods/patches.
Fallout series :3