Heres a little game that can be pretty addictive.

http://team.forsvarsmakten.se/english/#

It's an online game where 4 players connect to the same game. Once the game starts, each player must solve their own puzzle or question. With each puzzle/question solved correctly, a corresponding teammate gets more time to solve more puzzles. If a player is taking too long to answer his questions, or gets them wrong, his teammate will not be rewarded with extra time. Eventually, time runs out for that player and his game ends. Without time support from that fallen player, his teammate will eventually fall too. This domino effect happens till all players have 0 time left to complete their puzzles. Your game is then rated based on team achievement as well as personal achievement.

There is no sign up required so you can jump in at any time.

This is kind of an interesting game