Darren pointed me towards the London Freecycle Group. It's a straightforward mailing list where people (in London) post stuff they want to get rid of, the only condition being that it has to be free. It's also possible to make a request for something although the rules around this are a bit tighter. There's a lot of interesting things going through, and in the day I've been a member I've seen such things as:

• An old Dyson
• Some household doors
• Old videos (with and without films on)
• Some 'hardly used' felt tip pents
• A payphone.

It's quite high traffic but an excellent way of getting rid of something that you no longer want but someone else might.

London Freecycle Group
The main Freecycle site