Open House London - Have a look inside the various architectural jewels around London in September each year.
Time Out London - Listings and reviews for performance, arts, eating and drinking, and many other aspects of London life. This is the online version of the longstanding magazine.
londonart - "Shopwindow" for commercial art galleries, including works for sale in a wide range of media.
Major museums & galleries in London - including guides to collections, opening hours, online exhibitions and other resources:

City Heritage Guide: London - From the people at the 24 Hour Museum.
London Theatre Guide - What's on in London theatres. Includes seating plans.
London Dance - What's on in dance in London, including news, features and a directory of information for dancers.

London Chamber of Commerce - Part of the nationwide network of British Chambers of Commerce.
London & Partners - The inward investment agency for the capital.

KnowUK local information - Find services, leisure facilities, transport information and much more for your area [Can't get in?].
Westminster Community Information - Find organisations, services and clubs in Westminster.
Homeless London - Directory of resources across London. For more general, national information, see Homeless Pages.
Hot Courses - Details on an enormous range of courses, from part-time evening classes to postgraduate degrees. Floodlight is also a great database of courses throughout London.

Streetsensation - Entire streetscapes showing over 3,500 shops, bars and restaurants in the liveliest areas of London. Walk through the capital's best known shopping streets door by door.
City of Westminster Archives Centre - Your source of local history (focusing on Westminster & central London) information and help.
Survey of London - Extremely detailed histories from British History Online.

Map of London boroughs - from Hillingdon with links to their local government website homepages.
London.gov.uk - The web site of the London Mayor and the Greater London Authority.

The Evening Standard - London's daily newspaper. The day's news and sport, plus information about many aspects of life in London - restaurants, pubs, shopping, transport and local listings. Paid subscription required for detailed film/theatre listings and reviews.
Loot - Online version of the free ads paper.

London Datastore - An effort by the Greater London Authority to bring together the vast amount of data which is freely available from various organisations throughout the capital which covers a variety of subjects. There are also links to other useful sets of data. You can even request particular sets of data to be added.
Neighbourhood statistics from the ONS - Find statistics about the city, borough or ward by simply entering a postcode or name of an area provided by the Office for National Statistics and the 2011 census.

View London - A great site listing all sorts of facilities throughout the capital plus also including ratings and reviews.
Visit London - London Tourist Board site covering current events, transport information, excursions and more. Has a useful directory which includes 'essential information' (embassies, hospitals etc.)
London Town - London attractions, food & drink, accommodation, day trips & maps.
Accessible London - Photo access maps, which help visitors to navigate step-free routes around some of London's most visited areas.
TfL Journey Planner - Comprehensive journey planner for London, allowing you to specify places in several ways (e.g.: postcodes, stations). Click on 'More options' to be able to choose transport type, maximum walking time, disability options etc.
Visiting London - Travel Information - An informative site from TfL on London's buses (road and river), tubes, trams and DLR. Also has information on other modes of transport such as coach services, walking and riving in London and travel by taxi.