Historical version 6 of Bus Saver Tickets (view current version)
The bus equivalent of the Carnet you can buy of Tube tickets. Buy a book of six bus tickets from any newsagent displaying a Transport for London logo, and you end up paying just 70p for any bus journey (a saving of 30p per journey). 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 definitely the cheapest way to get around London.
Will generally be superseded by the pre-pay Oyster Card which you will be able to use on buses from around March or April (estimated).