http://openplaques.org/about

What is Open Plaques?
Open Plaques is 'the museum of the street'.

It is a community-based project started in 2009 which documents, catalogues and promotes commemorative plaques (often blue and round) installed on buildings and other public landmarks in the UK and beyond.

The service brings the people-powered history that plaques encapsulate to life - and to a larger audience - by making its website and database the definitive and most comprehensive resource about these historical markers. In turn, the data and resources produced by the project aim for wide distribution and re-use, and are available for free to all under a Public Domain declaration.

As well as telling you where they are, we aim to tell you who is commemorated on them, what those people are notable for, and what their connection is with the places where their plaques are installed. We've been able to uncover this data through a process of automatic natural language parsing and painstaking research by our team of volunteer co-curators (if you spot a mistake, tell us).

I think there is a blue plaque on the outside of the house used as 221B in the filming of Sherlock. It seems to be covered by a circular exterior lemon lamp to disguise it, but it is there if you look on google street view. It commemorates Guiseppi Mazzini.

According to the Open Plaques website there are 215 plaques in the US.

Thanks for posting Tobe it's a much clearer image than squinting at street view.