Two of my favorite apps, Facebook and Second Life, have come together with Andy Powell's creation of a Facebook application called "Second Friends." It allows you to see which one of your Facebook friends (who use Second Friends, also), are using Second Life!
First search in the application listings in Facebook to add the Second Friends application to your Facebook profile.
Then, in Second Life, you must visit the Second Friends kiosk on Eduserv Island in Second Life and touch it to obtain a secret key. The SLURL for the kiosk is:
Andy is planning on the incorporation of some other cool features in the future!