A Foaf is "Friend of a Friend" it's a very simple XML application that lets you list your friends and crawl around the chain.
Some Foaf's you may played on include the now defunct sixdegrees.com and friendster.
The new one is called Orkut. And it looks interesting.
I'm going to try to overly promote this. Unfortunately, there is a bug in their system that invite codes/email are not being generated. This makes it rather difficult to use.
I've sent two tech reports to the group. Not much of a response yet.
If you are interested, email me and I'll try to send out an invitation for you. Now note. On FOAF's I tend to 'friend' people I've met or interacted with consistantly. This means if you are a Net buddy, I might relegate you to a group labeled as acquaintance. Please, don't take that personally. It means, I'm still developing a 'friendship' with you. :)