London general:
Go London Downloadable walking tours
London Travel Guide (Simply London) As your London travel guide, we will help you 
plan your travel to London and suggest ways to enjoy your vacation
London Online All aspects of London life
A Visitor's Guide to London (London Toolkit) Logistical and practical information
for visitors to London  including public transport, airport transfers,sightseeing and accommodation. 
Walk London  Transport for London site.  Has details of six popular major 
walking routes including the London Loop and the Capital Ring

Off-beat London:
Hidden London Some lesser-known localities
London Footprints A variety of walks in the London area
Shady Old Lady Chirpy  site - hidden places  to visit in London
Quirky Travel  Unusual travel ideas for the discerning traveller
Going Underground The fun Underground guide

For those on a tight budget:
Do Free Stuff Free things to do in the UK
Frugal Living in the UK Money saving tips of all kinds - cheap recipes,
days out, shopping tips, finance, little freebies
London Freebies A variety of ways to save money - free stuff and special offers
London Net  We know you have a budget: Let LondonNet show you how to have fun 
without spending a pound. From art to music to sightseeing, there's plenty to do on the cheap
Voucher Freebies An impressive collection of discounts for a variety of goods and services  in  London.

UK general:
Britain Express UK travel and heritage The premier British Tourism directory - thousands of things to do and places to stay.  
NeverBeBored Search for great things to do and places to eat drink and stay in the UK

Review sites:
Randomness Guide to London Kind of database, kind of review site, used for
documenting interesting places in London
Trip Advisor Major review site for world tourism.

Looking after the Environment:
Carfree Walks Help save our planet! Walking routes in the UK without using a car.