The UK's Blue Plaque scheme is a way of commemorating the lives of famous residents of the country. Blue circular signs are erected on houses to indicate that someone of note was born or once lived there.
The PlaqueGuide is a Google Map of London's blue plaque houses. The map uses blue circular map markers to show the location of houses with plaques. You can find out who the plaque is for by just mousing over a marker. If you click on a marker you can view a Street View of the plaque and read a Wikipedia article about the individual commemorated by the plaque.
The PlaqueGuide is crowdsourced, so anyone can add information about the location of a blue plaque. The PlaqueGuide currently only shows plaques in London but there are plans to extend coverage to the rest of the UK.