The Collage Portal (Corporation of London Libraries and Guildhall Art Gallery image database) has historic maps of London. On "Advanced search" choose picture type as "map", enter place name of interest as search term. The website has thumbnail and part-screen images of the maps, and a facility to order a detailed image.
Great Britain Historical GIS Project at the University of Portsmouth (formerly at Queen Mary & Westfield College, London) has the Vision of Britain site which is very useful for historical maps of the boundaries of administrative areas in London, for example Registration Districts. They also have a London GIS with a number of statistical maps of London, including:
Detailed Bartholomew maps of London, Middlesex and surrounding areas as they were in the mid-20th century are available from Maps of London website.
The West Surrey Family History Sopciety have published a CD of Early 20th Century London: maps, street anmes and schools containing the six maps produced in 1906/7 by the London schools authorities and based on the 6" Ordnance Survey maps, but overprinted with administrative boundaries relating to the schools and the schools' locations. The CD includes a school index, a street index and the 'London County Council List of Streets and Places' of 1912 which gives all the streets as at that date, plus a list of abolished street names. Full details are on the WSFHS website under CD 16.