From steam i got Team Fortress 2(old veteran), Killing Floor (loadsamoney), Half-Life series ( play lots of mods, that are free,including Sven Co-op, Zombie Panic! etc...), Terraria (play it with bros, it's fun.), Warhammer (now playing Gold edittion). Fun fact :All those games were 5$ or less in steam (some in holidays sales). Steam does pay off, go get it.
Aside from multiplayer games, and some not from Steam, i play , and recommend of course: Star Craft 2 (with friends it goes from a super fun to a super awesome game, not mentioning the championships streams and solid game community), S.T.A.L.K.E.R series ( don't know these games? Search " Chimera Hunt Not for the faint of heart " on youtube),
Doom series (all of them beacuse RIP AND TEAR), Deus Ex series ( Now these got me by the heart, search " Morpheus Conversation " on youtube), Minecraft ( sssssssssss...). Among Others. And I'm sorry if this is not the right place to discuss games. Than again i just want to make some friends here.
May I also point out that this is reasonably by far the most welcoming sharing community that i've been to.
I wish steam would also sell Visual Novels... also eroges. :8