Have just purchased an Oystercard. What's an Oystercard? It's an electronic travel card for use on Buses, Trams, The Underground and some Trains in London area.
It has some advantages like online re-charging, re-use same Oyster card (it's actually plastic not card), not having to show all drivers you awful photocard :),
Then when you have got your Oystercard pop along to TheOystercard on ways to have fun with the card.