This is The Pay It Forward Foundation, founded by Catherine Ryan Hyde and J.P. Guerin of Tapestry Films:
http://www.payitforwardfoundation.org

This is the official Simon & Schuster site:
http://www.simonsays.com/author.cfm?isbn=0684862719

This is the official Warner Brothers movie site:
http://www.payitforward.com

This is the new site to chronicle the real-life movement:
http://www.payitforwardmovement.org/

These sites are not affiliated with the book or movie, but the author feels they capture the Pay It Forward spirit.

Favors Unlimited is a fabulous site where people Pay It Forward every day. If you are looking for ideas, this is bound to help:
http://www.favorsunlimited.com

Three great young people:

This is Katie:
http://how.to/payitforward

This is Ryan:
http://www.geocities.com/pay_it_forward/Pay_It_Forward.html

This is Brandon:
http://clubs.yahoo.com/clubs/payitforwardfoundation