If you are using ccbill or similar 3rd party processing on your paysite, sign each of your free sites etc... up as an affiliate under your paysite, then you will be able to send traffic via your linking code, thus tracking your clicks and sales.
If you are using ccbill and also are using the custom forms option, you can get by with just one referal account and then CCBILL's new system will track all the url's that sent signups.
Cheers,
BV