I think ANNO1777 is a best browser games, recommend it. First social game in which you can change virtual currency into real money. http://www.anno1777.com/index.php?i=175759
Might not be the kind of games you are looking for, but I like using Neave. You can play Frogger, Pac-Man, Tetris, Asteroid and other games for free.
http://www.neave.com/games/
I find the online version of Plants Vs Zombies quite good fun to play, and there's a new version of Bejeweled out December 7th as well, if you're into the kind of 'casual games' genre.