TubeMap app for iPhone

Anyone with an iPhone who is going to be in London: a must-have app is TubeMap. It's free, and gives you not only a map of the London Underground lines, but can also plot the quickest route from one Tube station to another. It works offline (so no wifi nor 3G needed) and has been invaluable to me. As an added bonus, when you're consulting it, it looks like you're just reading a text or something on your iPhone, and not obviously a tourist who doesn't know where the hell they are going. You're welcome.