Rosemary Morgan, one of the more active British genealogy bloggers, has started to put together a summary of the major London record repositories, local family history societies and online datasets, here.
It's a popular topic. The March issue of Family Tree magazine, includes an article on key research websites for London by Mary Evans. One site mentioned, not genealogical but certainly different, is http://www.derelictlondon.com/
The topic of London gives me a reason to give a plug to the BIFHSGO conference this September which will have a London and area theme. The program will be posted soon at www.bifhsgo.ca