Gamers, I need your help!

[blehmeh05]

So, I'm starting up a new research collaboration and we're designing an experiment where we need to have participants come in and play a game in separate rooms for a little while. We're looking for a relatively simple online game (no deep RPGs or RTS types) that has both co-op and vs. modes (the idea is for some participants to develop a sense of community as they play with someone in the next room while other participants develop a sense of competition) and is either free or relatively cheap.

Any of you guys know of something that might fit the bill? A puzzle game or simple arcade style game would probably work great, but I'm having trouble finding one that has both cooperative and vs. modes.

All suggestions extremely welcome!