The London "Freedom Pass" is by far and away the best deal. It is valid on all London buses, tubes, Overground, DLR and trams 24/7. In addition it is valid on almost all local bus services across England (it has the English National Concessionary Travel Scheme (ENCTS) 'Rose' printed on it to show this). This concession is usually limited to after 9:30am weekdays. It is also valid on almost all local rail services within the London area (but again limited to after 9:30am weekdays). In addition holders get a 50% discount on most riverbus services and a 25% discount on the Emirates "Airline" cablecar service.
At present it is issued to people who have reached women's State pension age but people over 60 but under that age can apply for an Over-60s Oyster. For an administration fee of £10 they will be provided with an Oyster card which provides all the concessionary travel of a Freedom Pass but only within London.