If you need a map of London for planning purposes, Google Maps is definitely the best tool. Not only you will see the map itself, but also will be able to see sightseeing, restaurants, shopping and other places of interest on the map. Use "Explore This Area" and "Search Nearby" options.
London Underground Map
The Transport for London website is one of the best websites in the world for planning public transportation trips. They have a great route planner. But if you are looking to download London Underground (or Tube) map, you can also download it from TFL website.
Public Transportation Maps
Underground, Bus, DLR, National Rail, Tram etc. - download the maps from TFL website.