You can try Jrox Affiliate Management Script? (link to my review), it's free till you attact 250 users.
Don't just start with a free one because it's free, unless you plan on keeping it afterwards. Otherwise you risk losing most of your affiliates when you switch. Do the homework first - invest some time comparing and then pick what's best for you. Cheers, Peter
Its worth spending some money on affiliate software. I can't remember the name of the one I used to use, let me see if I can find it... UGH! It was part of a subscription process. $199 per year and you had access to all of their scripts. If I can remember I will post the name. NM: http://www.groundbreak.com/ Ultimate Affiliate. Its worth every penny.
I'd actually opt for Affiliate Pro but I have more things to do than handle the affiliates full time so I consulted with friends and they said why don't I try a dedicated third party services. It's more convenient, even cheaper.