Under Ground

I found this very well done Tube/Underground/Subway/Metro App for IPhone and IPad some time ago. Really helpful not only for New-comers and Tourists.

London Tube Delux

You can plan your journey by tapping your start and destination on the map or by typing them in, locate your nearest station, departures and last trains. The only drawback: one hardly ever has a signal Underground.

TubeMap

Also very good is this TubeMap App, and it's free.