Subway Navigator

Maps and stations of subways and other heavy rail transit systems throughout the world, and a route finder detailing the path between any two stations.
http://www.subwaynavigator.com/

The Subway Page

Maps for subway transit systems from a number of cities all over the world.
http://www.reed.edu/~reyn/transport.html

UrbanRail.Net

Information on subway systems organized by continent, and news, links, literature, and a gallery of general subway information.
http://www.urbanrail.net/

TubeMap.com

Provides direct links to route maps from subways around the world.
http://www.tubemap.com

Chris's Transit Pics Website

Showcases photographs of subways, buses, railroads, and monorails from a variety of locations. Also features transportation information for tourists in New York City.
http://www.transitpics.com