Whilst the iPod touch is an useful device, the iPhone can be very useful as it has the connectivity that makes some applications work better. One of these is London Tube Deluxe (iTunes Store link).
If you travel by tube in London and it can be useful to know when and if there are problems on the Tube. Combine that with the tube map and using Google Maps to work out where alternative tube stations actually are in London, it is possible to ensure you can get where you need to get to even if there are terrible delays on the tube.