Actually Transport for London, which includes bus, bike, foot, boat, and other options for getting around the London area. The TFL Journey Planner is a fantastic resource.
It has been mentioned elsewhere, but I'll repeat the advice: get an Oyster card when you arrive. It's faster and cheaper than other ways to pay.
Egotist: A person of low taste, more interested in himself than in me. -- Ambrose Bierce