I should have mentioned MapTube long ago; Andrew Hudson-Smith wrote to me about it in May:

MapTube, the new mapping site from the guys at Digital Urban and CASA at University College London to view, overlay, mix and match choropleth maps, now includes a free creation tool.

Google Map Creator provides a quick and free way to convert .shp or formated .csv for viewing in Google Maps and MapTube — allowing datasets to be quickly and easily shared and visually compared against any other data on MapTube. In short it is perfect for local councils, government organizations, academics and general users who want to view and share their data in the easiest way possible.

Getting choropleth maps onto Google Maps has been a longstanding challenge; anything that makes it easier for people or organizations to get such data online is by definition good. So far, 134 maps have been added to MapTube (you have to host your own map tiles and data); more than a third of them deal with London. There is a dedicated MapTube London page; its launch in June was covered by the Nature Network, which also talked about MapTube in general (thanks to Richard for that link).