Battle.net RealID

http://us.battle.net/en/realid/

In spite of all the hooplah about using real names on the forums, one thing that RealID does EXTREMELY well is makes it way easier to find people you know who are playing the game. ┬áBecause you can view the friends of people in your friends list, you can easily find mutual friends that you didn’t otherwise know played, and adding them is very simple.
I know more than once I’ve been on Starcraft 2, and none of my friends have been on, so I browsed THEIR list of friends to see if I knew anyone, added somebody, and boom, someone to play with that I knew!