I'd say develop it yourself. It only took about a day in rails. - make a migration to add "referrer_id" to users table - put a before filter in application_c

I'd say develop it yourself. It only took about a day in rails. - make a migration to add "referrer_id" to users table - put a before filter in application_controller to watch for a special param used in affiliate codes, if it exists store in session - when new users sign up set their referrer_id out of the session if it exists - whenever the "action" is taken by the user that triggers a referral commission, put this is pending transactions - use a monthly cron job and paypal gem to send affiliate commission each month That was the gist anyway.