Amsterdam.info
Cultural events and festivals in Amsterdam.
http://www.amsterdam.info/events/

Le Cool
Free, weekly local events guide that is delivered direct to your email box.
http://www.lecool.com/amsterdam/

Rick Steves'
A list of festivals in Belgium and The Netherlands.
http://www.ricksteves.com/plan/festivals/benfest.htm

Time Out
Events guide to Amsterdam; updated weekly.
http://www.timeout.com/amsterdam/thisweek.php