Technology + Mass transitTransport for London provides open live data of where all its trains are at any time. This has allowed smartphone app makers to build really cool apps that give you a live view of the whole transit network, one of these being Tube Map Live, a free app for both iPhone and iPad (the same people also make Tube Tracker, a more full-featured app, but it costs £1.99).
If you want to plan trips and track specific trains, Tube Map Live isn't for you. But if you simply want to play around with a live map of the London Underground and once in a while look up where a train is, this is perfect.
As a nice bonus, the aesthetics are really good.
Try it out for youreslf: Tube Map Live