There are a number of ways you can make money out of cashback sites beyond the usual cashback on shopping that all cashback sites offer:

Some cashback sites offer you money for referring your friends. This can be a fixed amount or a percentage of the cashback they earn (not taken from their earnings).
Cashback White Label

You can have your own cashback web site. The provider will provide the back office payment processing and customer service leaving you to do the job of marketting your cashback site.
Networking Links

Internet Cashback offers the ability for you to use links in your social networking circles that will earn you all the cashback on someone elses purchase.

British Rewards is a UK cashback site offering cashback at over 1,000 online shops. Get money back every time you make purchases online and get paid to take part in competitions & more.