Affiliate software

AWIZ Affiliate Manager - A comprehensive and robust affiliate software program. The main difference from other products is that you buy NOT a script, but your finished partnership project - with a design, fully adjusted, tested and operational product!
MPA3 Affiliate Software Program - MPA3 from Mansion Productions is a comprehensive and robust affiliate software program that allows merchants to grow and monitor their online businesses accurately and expeditiously.
NATS Administration & tracking system - Advanced affiliate administration & tracking software with a very large amount of options.

Here are some articles about how to start an Affiliate program:

Affiliate Program Basics: 1 (XBiz tutorial)
Affiliate Program Basics: 2 (XBiz tutorial)