Chess.com - http://www.chess.com/ The #1 place to play free online chess and learn chess rules, openings, & strategies. You can also find chess games, explore downloads, & review chess sets.
Escapa! - http://members.iinet.net.au/~pontipak/redsquare.html A game. Elude blue rectangles as a red square.
ESP Game - http://www.espgame.org/ The ESP game uses images to create a game that as you play, lables images all over the internet to enhance search results. What a novel concept.
Kingdom of Loathing - http://www3.kingdomofloathing.com/login.php The Kingdom of Loathing (or KoL, as it has come to be known by its player base) is a free, comical RPG, brought to you by the folks at Asymmetric Publications.
The Red Button Game - http://www.coolbuddy.com/games/The_Red_Button/The_Red_Button.htm Shoot the bad guys to change history for the better.
You Don't Know Jack - http://www.youdontknowjack.com/ Quite a twist on a trivia game. This one can sometimes make fun of you. Odd questions make answering things you already know a bit more difficult. I've been playing for over 10 years.