Best known as a phone app it's also a website so you can plot routes beforehand and play around with it https://citymapper.com/london and see also their How to Survive the August 5-6th Tube Strike advice which tells you about the adjustments the app's made to discount non-functioning tube bits and highlights the cycle hire system as a Plan B.
The (open) data comes from TfL who of course have their own Journeyplanner https://tfl.gov.uk/
Google Street view
If your journey involves walking in unfamiliar bits you can rehearse the journey by using Google Streetview - that's where they've driven around London and photographed everything and then knitted it all together to form a 3D view of London.
Go to Oxford Street, wait for the page to load and spot the little person icon in the bottom right. Click and drag it onto one of the roads paying careful attention to the little green circle below it, that's what you're lining up with the road you're interested in. Drop the little person and see what they see, you can then walk through the roads online.
Find a bus
All bus stops in London have a 5 digit code which can be appended to the following unfinished address to bring up the Countdown page to tell you when your bus is due.
Web example: http://countdown.tfl.gov.uk/#|searchTerm=50082
Mobile example: http://m.countdown.tfl.gov.uk/arrivals/50082
Rail departure boards
Find out what your nearest railway station's three letter code is (eg Blackheath is BKH) and add it to the end of this unfinished web address to see when the next train is - works for all UK rail stations, not just London! http://ojp.nationalrail.co.uk/service/ldbboard/dep/
- Example - Blackheath: http://ojp.nationalrail.co.uk/service/ldbboard/dep/BKH/
- Example - London Bridge:http://ojp.nationalrail.co.uk/service/ldbboard/dep/LBG
- Example - Cannon Street: http://ojp.nationalrail.co.uk/service/ldbboard/dep/CST
- Example - Charing Cross: http://ojp.nationalrail.co.uk/service/ldbboard/dep/CHX/
- Example - Liverpool Street: http://ojp.nationalrail.co.uk/service/ldbboard/dep/LST
Thames Clipper ferries
They'll probably be every bit as busy as everything else but if your travel involves the following places it might be worth looking into. If you have anything travelcard-like you can get ⅓ off the cost.
Find your pier
- London Bridge City Pier (London Bridge Walk, London SE1 3UD)
- Chelsea Harbour Pier (SW10 OXD)
- Waterloo: London Eye Pier (Westminster Bridge Road, City of London SE1 7PB)
- Embankment Pier (Victoria Embankment, London WC2N 6NU)
- Blackfriars Pier (Victoria Embankment, City, London EC4V 3QR)
- Bankside Pier (Bankside, London SE1 9DT)
- Tower Millennium Pier (Lower Thames Street, London EC3N 4DT)
- Canary Wharf Pier (Docklands, London E14 8RR)
- Surrey Quays: Greenland Pier (South Sea Street, London SE16 7TE)
- Greenwich Pier (Greenwich Promenade, Greenwich, London SE10 3QT)
- North Greenwich Pier (Peninsula Square, Greenwich, London SE10 0PE)
- Woolwich: Royal Arsenal Woolwich Pier (Royal Arsenal Woolwich, Woolwich, London SE18 6TL)
- Millbank Pier (Millbank, Westminster, London SW1P 4QP)
- Putney Pier (SW15 1JW)
- Wandsworth Riverside Quarter Pier (SW18 1LP)
- St George Wharf Pier (Riverside Walk SW8 2LE)
- Chelsea: Cadogan Pier (Chelsea Embankment, London SW3 5RQ)
- Isle of Dogs: Masthouse Terrace Pier (Napier Avenue, Isle of Dogs, London E14 3QB)
- Hilton Docklands Riverside (265 Rotherhithe Street, London SE16 5HW)