There's been many web games over the years from MMO's to online variations of Tetris, so I'm surprised there's not an active thread dedicated to them (Maybe nobody plays them anymore?) Anyways, this thread is dedicated to any cool web based games you may have found, from big to small and anywhere in between.
Here's one I found today (saw it suggested in Edge magazine.) It's rather unique and blends multiple genres together in one game.
http://christopherwhitman.net/games/Run/play/
Enjoy!