Frommer's
Contains information about the climate in Amsterdam and the best time to visit the city. http://www.frommers.com/destinations/amsterdam/0043030002.html

Hotels Holland
Tips on what to wear.
http://www.hotels-holland.com/amsterdam-info/adam-6weather.htm

Luxury Travel
Features a chart detailing the average rainfall and temperature in Amsterdam throughout the year.
http://www.luxurytravel.com/cityguides/amsterdam/weather.html

Wunderground.com
Current and forecast weather information for Amsterdam, as well as information about the length of the day and moon phases.
http://www.wunderground.com/global/stations/06240.html

Yahoo!
Provides comprehensive and up-to-date weather information on Amsterdam. http://uk.weather.yahoo.com/NLXX/NLXX0002/index_c.html