title: gems/plugins for affiliate system?
body: <p>Do you remember any gems/plugins out there to support an affiliate (referral, partner) like system? </p>
  
  <p>e.g the 37signals <a href="http://37signals.com/svn/posts/1063-launch-announcing-the-new-37signals-affiliate-program-with-recurring-income" rel="noreferrer">affiliate program</a></p>
  
  <p>Any help/links would be appreciated.</p>