Visualiser is a tool to graphically explore your facebook social network. You can use it to see how all your contacts are connected to each other, and in the process disclose unknown common friends. Other FOAF (friend-of-a-friend) disclosure would be interesting, but unfortunately, the Facebook API doesn't include this important bit of data.
The tool allows filtering your network by Gender and Relationship Status (single, engaged, etc). You can also choose to see the inner connections of your facebook network and play with the distance between nodes (friends).