iDevAffiliate is a powerful php-based affiliate tracking system. It supports several platforms and upgrades are included.

iDevAffiliate works with nearly all merchant accounts, shopping carts and content/membership management systems.

iDevAffiliate recently released Commission Alert, an affiliate and admin alert add-on module, which can increase affiliate sales motivation. iDevAffiliate can also be hosted by iDevDirect for a simple monthly fee. This listing below is for the installed package (one time cost).

LIMITATIONS: Supports only 1 commission scheme for ALL products, max 2 tier levels, source is provided but encrypted so customization is not possible.
See iDevDirect site for details and online demo.

Alternatives: Compare with Post Affiliate Pro and Jrox