APTI
A guide to the Sneltram, Amsterdam's metro, that includes a map of the network.
http://www.apti.is.nl/metrolne.html

Metro Planet
Contains unofficial, but detailed, information about the city's metro system featuring maps, pictures, links and practical tips.
http://www.metropla.net/eu/ams/amsterdm.htm

Tickets, please - Hotel Prinsenhof
A guide to using the Strippenkaart, which allows you to take multiple rides on Amsterdam's buses, trams and metro system.
http://www.hotelprinsenhof.com/mokum/gvb.html