Invitation Plugin?

I’ve seen several invitation plugins that create registration codes, close registrations to everyone but those with codes, allow members to invite other members by adding friends’ email addresses to a form and sending an invite, and grabbing a member’s contacts’ info from their Facebook, Twitter, or IM friend/followers, but I need something that generates a referrer URL for each member that they can give out freely (post in forum signatures, as a Facebook status update, etc.). I also need this system to keep track of how many registrations are actually resulting from each referrer URL.

Does this exist anywhere?