Developing a referal system can be a great marketing tool. Allopass, a company servicing micro payments managed to raise to €20Mo valuation very rapidly thanks to a clever affiliation scheme:
Generate Lifetime Earnings:Install an Allopass banner on your site.
When a new merchant sign up with Allopass via this banner, we give you 10% of the revenue generated by this merchant for the life of your account and his account.
http://www.omnistaretools.com/omnistartell/ Seems an interesting PHP package, and first time I come across an open source solution for managing this type of marketing channels. Worth a try.