I’ve written an extension to integrate Magento with Post Affiliate Pro. The integration is free. PAP starts at $49 (one time fee) which is more than reasonable. We scoured the web looking for good affiliate software and PAP was the best we found when we considered features and price. You can get the integration via Magento Connect, or via direct download at the link below.
Details on Post Affiliate Pro and the Affiliate Module
Affiliate Module on Magento Connect