AmsterdamBy.com
Lists major operators of walking tours in Amsterdam.
http://www.amsterdamby.com/sightsee/sight5.htm

Frommer's
If you want to discover Amsterdam on foot by yourself, then check out this detailed guide listing several different routes.
http://www.frommers.com/destinations/amsterdam/0043020033.html

Simply Amsterdam
A comprehensive index of guided tours in Amsterdam, walking tours included.
http://www.simplyamsterdam.nl/services.htm