Mashable has done a nice writeup of some really cool social graph visualization tools for Facebook. The social graph is an incredibly powerful networking tool. If you don't know what I am talking about, I'll leave it to you to figure out ;-)

 6 Gorgeous Facebook Visualizations

Nexus is one of the most interesting Facebook apps I have found. It maps your Facebook friends and their connections with each other. Mouse over a node to see who knows who.
This static image is my Facebook social graph... hit the link to see yours!