One of my personal favorites is http://hop.clickbank.net/?instantnet/clickbank -- they give you some good exposure for your affiliate program and handle commissions, etc. automatically with timely checks. Great service. I think the only charge is a very small setup fee, but not sure. Another popular option is CJ.com, although I believe only larger sites use their program since it costs thousands of dollars to setup. But, if you have the money, it IS a great affiliate network.