Remarkably useful and simple
As a visitor to London from outside the UK, mastering the bus system is a challenge for me. This app provides a great solution: a detailed map of a large part of London over which bus routes are superposed. The route has each stop on the way marked with a dot, which will give you the stop name when you touch it. The map is clear enough to show where each stop is (try working out which bus stop at Trafalgar Square is the right one otherwise) and since it lives in your phone you don't need an internet connection to access it. All this takes a fair bit of space on your phone, but it's worth it.