FREGAL ONLINE GAMES (UPDATED 2013)

A good place to start is using steam game client: http://store.steampowered.com/. Here's a list of all the free games they provide: http://forums.steampowered.com/forums/showthread.php?t=851573. The best one is dota2 & team fortress2

Another list based on graphic and epicness:
https://www.planetside2.com/
https://www.pathofexile.com/
http://world.needforspeed.com/
https://www.dcuniverseonline.com/
http://pangya.gamerage.com/
http://maplestory.nexon.net
http://audition.nexon.net

and the more casual games u can find in facebook games or this list:
http://www.popcap.com/games/plants-vs-zombies/online
http://fallenlondon.storynexus.com
http://www.dampgnat.com/wonderputt
http://continuitygame.com/playcontinuity.html
www.newgrounds.com/game
www.miniclip.com