If you want a London walk never to forget, try this list of guides:

http://www.blue-badge-guides.com. The list accredited official 'Blue Badge Guide' for the city. You tell them what part of the town you want to know about and he/she researches it and walks and talks you through history, architecture, culture and the arts. You come to the city as a stranger and leave a Londoner.

http://www.soundmap.co.uk  London Audiowalk -You download them straight to your MP3 player and get led around by local narrators who show you inside the real london. Soundmap walks cover: Soho, Brixton, Camden Town, King's Road and Sweeney Todd

Free Tours by Foot- offers free London walking tours as well as food and bike tours.   The tours have no upfront cost and operate on a pay-what-you-like basis.  Tours include the Westminster Tour, the City of London Tour and many others.  

Rambles from the London Tube

Definitely different! London Tube Rambles is an offbeat site exploring  Outer London with over a hundred free self-guided walks, all starting within a mile of an Underground station.The site is fully searchable by destination or by type of walk and includes a list of quirky things to discover. As well as urban routes there are plenty of medium length waterside walks, plus some longer rambles in country surroundings.   You’ll be surprised where you can get to by the Underground (a lot of which is in fact ‘overground’.) 

http://www.londongreeters.org. Part of the Global Greeters Network this site offers free personalised guided tours of a number of London's neighbourhoods.