I created my own signup system. It's entirely PHP/MySQL, with email verification to cut down on the spam without being obnoxious to use.
Though it currently lacks automation into the server-side control, I think it better to have that step done manually. It really cuts down on the amount of abusive clients- as many of them thrive on instant activation, which manual creation does not provide.
Mine also supports Paypal IPN, automating the accounting aspect of it. The first payment is checked manually at a couple of points in the system, and after that it remembers the data and is able to match incoming payments to the customers that sent them.
Possibly someday I might streamline it and make it GPL for others to use, but for now I am satisfied with what it's capable of for my own needs.