I've had a few maps links building up for a while and Dave has prompted me to get on and post them by urging his readers to vote for Harry Beck's London Underground Map, which has made it into the top 3 of the BBC's Great British Design Quest. The
Transport for London site offers the tube map in many translations. And ooh, look - you can get a London Underground Map mouse mat from London's Transport Museum!
I have a lovely book about the map's development - Mr Beck's Underground Map by Ken Garland. I see there is now a 'sequel' - Underground Maps After Beck by Maxwell J. Roberts. Then there are Mark Ovenden's Metro Maps of the World books (1 and 2).
Elsewhere, 30gms is disappointed by an attempt by The Guardian's Dorian Lynskey to chart the branches and connections of 100 years of music using the London Underground map (pdf of that map here), and links to Geofftech's Tube Map Variations and Spin-Offs. Boing Boing posts about anagram transit maps, and has examples for Miami, Dublin, Ontario, Dallas, Glasgow, Portland, Ottawa and Houston.
Check out the amazing map of the Moscow subway - brought to my attention by the BBC's Map Man telly programme. Further Googling has turned up maps for Sydney, Glasgow ('The Clockwork Orange') and Helsinki.
And of course, I can't finish without mentioning a favourite reworking of Harry Beck's tube map - Simon Patterson's The Great Bear.