Update 2006-02-01: User Referral Module has been released for 4.6. Get it from Drupal.org.

A client recently commissioned a user referral module whereby an existing user of a site can refer another user and get some sort of credit for it.

The module is conceptually similar to the affiliate module, but instead of a "site-to-user" model, it is a "user-to-user" model.

Users in designated roles have a referral link that others can click on to register to the site. Once the new user registers, a database link is recorded as to which user referred them.

If the user points is installed, the referring user is credited with a certain number of points. If Adsense with revenue sharing is installed, an admin assigned percentage of page views for nodes created by the new user is assigned to the referring user.

All this provides more incentive for users to signup more users and hence the site grows exponentially. 

Admins can view a summary or detailed report of who referred who and when. 

I plan to eventually release this module to the community soon.

This module compliments nicely the suite of site tools that I wrote that help build a community, such as Node Vote, User Points, Adsense with Revenue Sharing, and Click Thru Tracking.