Moscow Metro boasts "the biggest photo collection on the web" of Moscow's impressively opulent subway system. K and I have often discussed the pros and cons of aesthetically pleasing subway architecture, and he would always tell me about the surpassing extravagance of Moscow's subways...I could never imagine what it was like until I saw these pictures. The site features beautiful panoramic images of twenty amazing stations around Moscow.
Then, of course, there are maps. Animals on the underground is a great site displaying images of animals traced out on the lines of the London Underground map. In addition to the pictures themselves, they have desktops, games, and merchandise. The elephant below is perhaps their most classic creation.
A more recent discovery is that this same trick can be performed with maps of Germany's various subways: Zoobahnplan is the colorful result. The site is in German only, but it shouldn't matter.