Eurolines
A coach network that covers The Netherlands as well as the rest of Europe.
http://www.eurolines.nl/

BUG Europe
Provides an overview of coach and bus travel in Holland.
http://www.bugeurope.com/transport/busnl.html

Busabout Europe
A hop on, hop-off backpacker bus service that makes a stop in Amsterdam.
http://www.busabout.com/