Affiliate tracking software

Does anybody know of any good affiliate tracking software?
It would ideally
need to be able to have customisable sign up information
cookie tracking
paypal integration
override system
php based, or python no perl or ASP, java though

oh and be free! or a good price if not free!

I was looking for a script like that, but never found something good.

Let me know if you find anything :)

http://www.qualityunit.com/postaffiliate/
http://sonicfog.com/index.php?option=content&task=view&id=21

seem interesting will test them out

Wow those look great and they are open source.

I am impressed :)

Affiliate tracking comes with most e-commerce tools....right?