TAG: It's a first person puzzle solver, kind of like portal.
Pretty fun, short though. info here
And if you've got steam you could always sift through their selection of mmorpg's.
Star Trek and LoTR online are pretty epic, if you're interested in those things.
If you don't already have it, download steam and look at there free selection of games. Other than that, there are also many flash games that don't require installing anything, but it all depends on what type of genres of games you play.
League of legends is one good/free game. You can get basically what real money can buy in game so if you are not up for paying to get stuff LoL is one free game.
Rumble fighter
Zone 4
Tactical Intervention
Maple Story
Runes of Magic
Warframe
Brawl Busters
Microvolts
Ace of Spades Classic - http://buildandshoot.com/ (I personally hate both versions)
Dwarfs f2p
District 187
Crossfire (For Nostalgia)
M.A.T online
LoL (If you want to play a really bad copy of DOTA)
Team Fortress 2
Quake Live (Quake 3 repackaged basically)