Post affiliate pro has a free version.
Anything that's free is going to have some limitations though.
Free Affiliate Software
And from an affiliate viewpoint, no serious affiliate would work with a merchant that was using free software because I'm afraid they would see it as a lack of commitment to the affiliate channel or think "if they can't afford to buy software then how do I know they can afford to pay me?"
So you get what you put in. If you don't put much into the affiliate channel, in all honesty you probably won't get much back out.