The Open Guide to London: the free London guide

The bus equivalent of the tube Carnet. Buy a book of six bus tickets from any newsagent displaying a Transport for London logo, and you end up paying just 65p for any bus 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.