It's not nearly as hard to find helpful websites for London as it was for Venice, Rome, and Paris because in London they speak English (duh). Many museums in London are free too, so any budget savings are mostly to be had in the dining and accommodations areas. Hotels were so expensive in London that I couldn't find anything from the usual sources Trip Advisor, Expedia, and newspaper review sites. I ended up finding a boutique B+B in the Belgravia area (cha-ching). It's cute, modern, has 24-hour tea service, breakfast, and free loner bicycles.
Here are a few links that we found useful. We also used Rick Steve's London guidebook a bit. Please post your useful links in the comments.
Planning Your Time:
The London Pass itself is unnecessary as most of the museums in London are free. It just wasn't cost effective for us. That said, the London pass website is a great place to start planning a trip to London with its interactive maps and other features.
The London Guide has a great list which they call Combining Attractions. It groups together sites which are within walking distance of each other and accessible off of the same metro stop.
London Walks offers a variety of tours priced at £7 ($14 USD). These are guided trips on foot that last roughly two hours. Most leave in the vicinity of an Underground stop. Look for a guide holding up the company's white sign. A good way to skip the lines at the Tower of London and a few other sites.
How to Get Around:
How To Save Money on London Transport
How to Recognize Bus Stops
London Tube Trip Planner
Gatwick Airport: Getting from Gatwick Airport to London, Heathrow, or other UK cities by train (Gatwick Express), coach service or car service.
Heathrow Terminal 3 Check-In Advice
Heathrow Express train transfer to central London website, cheaper than a cab, more expensive than the tube, faster than both
Eurostar Website: Info and booking engine on the train that nearly 22,000 people take between London and Paris every day.
Theater tickets are way cheaper in London than in New York, even with the exchange rate. This website has an event guide which will tell you what is playing while you're there.
Attraction Official Websites: Go to the source for the most up-to-date information, dates of closure and special exhibit information.
Changing of the Guard at Buckingham Palace-View the schedule here.
Imperial War Museum's Cabinet War Rooms
London Design Museum
Madame Tussaud's London Wax Museum
Museum of London
National History Museum
National Museum of Science and Industry
National Portrait Gallery
Sherlock Holmes Museum
Victoria and Albert Museum