Tube Map for iPhone is a fairly self-explanatory app - it is a maps of the London tube. The scrollable map does not require an internet connection so you can access it even when you are without signal.
While this is useful, the app does have more to offer. Tube Map can also provide information about potential problems on any of the lines so you can consider taking a different route.
To this end, Tube Map also includes a route planner than can be used to calculate the easiest way to get from A to B with the fewest changes. The route you need to take is clearly displayed on the map but you can also view changes as a list.
If London is unfamiliar territory, the app can also be used to quickly locate the nearest tube station. Whether you are a seasoned tube traveler or just visiting the capital, this is a wonderfully useful tool.