I'm sure you have all heard of Quake Live.
It's an FPS that you can play with just your browser software. It will be free on the web. Sign up to play the beta here:http://www.quakelive.com/
Read here for more: http://en.wikipedia.org/wiki/Quake_Zero
This is a MP game that should/will attract millions of players. The intent is to apparently make Quake Live a "major league" FPS eSports competition game. Possibly an attempt to overtake CS.
Sadly, Activision doesn't seem to want to take CoD in that direction and is willing to leave the eSports arena to the next guy.