Best website I know for freegames is brettspielwelt.de/
(is the English version).
It's a company that makes a lot of board games, that offers most of them for free online (albeit without expansions). You have to have java to play, but it is free (no registering even). Download the client, and it basically is almost instantaneous. Very easy interface for most of the games.
These are some of the games they have:
Dominion, Pandemic, Wizard, San Juan, Puerto Rico, Power Grid, Bonanza, Tichu, Settlers of Catan, Caylus, Can't Stop, Take 5, Carcassonne, Cafe International, Backgammon, and Stone Age.
You play against people around the world. Pretty multicultural (Germans for the most part, tho).
There's also another nice site for one of my favorite games
Ticket to Ride is basically a train tycoon game, but a very easy and fun one to play. It really helped me with my geography, too.