Hey try this http://sonicfog.com/sonic_affiliate.php Tracks clicks and sales. It is free but chage $50 if you want it installed.
Post Affiliate Pro has a free version. Although I'm not sure how many strong affiliates would join a program that seems not to be willing or able to invest in their affiliate channel. It would probably be fine if you just want to set up a little referral program and don't expect much out of it. My 2 cents...
Some of the low cost ones I hear frequently mentioned are idevaffiliate, post affiliate pro and groundbreak. I can't vouch for any of them personally, however if you do a comparison between those 3 you should be good.
If you are considering Post Affiliate Pro, I would strongly recommend that you first try to contact the company involved (QualityUnit.com), and ask a pre-sales question or two. My own experience with PAP is found at: http://forums.digitalpoint.com/showthread.php?t=1128989&highlight=Post+Affiliate I too started with Post Affiliate Free -- however, the tracking options are very limited and I later decided to upgrade. Because I was unable to obtain PAP Pro 4, I purchased iDevAffiliate yesterday (still figuring it out, but so far I'm impressed both by the software and by the support)