The others mentioned are great.

What we have used if we want lots of control etc.. is Post Affiliate Pro. its a pretty robust affiliate toolbox.

If we just need simple - I have actually found the amember built in affiliate program to be sufficient as well if you are already using amember for your memberships. (and it has 2nd tier)

there is of course clickbank as a processor/built in affiliate system, but that is only good for certain types of niches.