I would recommend idev affiliate. It is definitely not free, but well worth the cash.
Sorry, not true. Idevaffiliate does not support their product. I got a refund and people are complaining about poor service on forums.
There are 2 free affiliate software I know about, though none is really free.
jrox.com - installed on your website, free up to 50 affiliates I think.
Affplanet.com - hosted - free until you reach certain amount of transactions.
Note that switching between solutions is not easy and not cheap so I suggest you make serious research on software you choose..