Post Affiliate Pro is by far the most powerful affiliate program software I have ever seen

Post Affiliate Pro gives you the ability to create and customize your affiliate program to your heart content. You can use the “language editor” to change anything you want inside your merchant or affiliate members panel. You can run pay per lead, pay per sale, and even pay per click campaigns. It is very robust and am continually been blown away by the amount of customization they allow us to do.