Heritage routes 9 and 15 operate every day between Royal Albert Hall to Tower Hill via the Strand, running every 15 minutes for the same price as a normal bus fare.
Granted, these are thoroughly tourist routes but nevertheless it is brilliant to see the old buses working their way through the middle of town, and there is a certain amount of nostalgia about catching the old style Routemaster which makes it feel like a truly London experience
For those who have forgotten, you simply hop on at the back and then wait for the conductor to come and collect your fare, either in cash or by Oyster.
There isn't a great deal of information on the tfl website, but it's only a bus, so how much information do you need? There are timetables on the website though, just search for route 9 or 15.