Referral, or better known as affiliates are quite a risky business setup as you have to rely that any person you send to someone else that you would be paid for it. As I am sure some may tell but affiliate marketing is where quite a lot of income is made (and myself included).
While I have little interest in reviewing any affiliate services, the only one I am going to review is the one that is found at Co.CC. I have written about Co.CC before (such as: How to make the perfect e-mail address), have used it myself for so many throw away websites or “in creation / trial” websites (even Just a Community and Post2Trade used Co.CC at one point) and can personally say that Co.CC is a very good service.