Historical version 8 of Bus Saver Tickets (view current version)
The bus equivalent of the pre-bought Carnet you can get for Tube tickets. Buy a book of six bus tickets (�6.00 for an adult, �2.10 for a child) from any newsagent displaying a Transport for London logo, and you end up paying £1 for any bus journey (a saving of 20p per journey on cash fares, but 20p more expensive than Oyster card pre-pay).
Just tear off the hexagonal portion as you board a bus and give it to the driver (or if it's a Routemaster, wait for the conductor to come around).
TfL have a list of newsagents selling these tickets (and a range of others).
For the occasional public transport user this is no longer the cheapest way to get around London - go for the pre-pay Oyster card instead if you can.