May favorite is Lee Jackson's Dictionary of Victorian London, not just because of the excellent content, but because it's FUN. I can also highly recommend his blog The Cat's Meat Shop and following him on Twitter @VictorianLondon is very entertaining.
The site I have only just discovered is the London Miscellany section of map.thehunthouse which is really useful for researching streets, with its very helpful lists of street name changes, and some maps. There are even more good maps at MAPCO and MOTCO.
The Charles Booth Online Archive at the London School of Economics not only has the famous colour-coded Poverty Maps, but also the survey reports and notebooks that go with them. They are particularly good for the East End, but there is plenty of good material for the rest of London, too.
If you want pictures of London back to 1500 (and even more maps) there is the Collage collection from the London Metropolitan Archives and Guildhall Art Gallery.
For printed sources, particularly on the early history of London, there is a large section devoted to London and Middlesex at British History Online
Anyone who lives within easy reach of London, or who visits regularly might want to join London Historians but even if you a long way off there is plenty of interest on the website, and a blog to follow.
And finally, some pictures, because I like to share them.
|Billingsgate Market 1841|
|Kings College Hospital 1872|
|London Bridge and Dyers Wharf c1750|