They were started in 1867, with the thought to commemorate someone famous who had lived/worked/died in that building.
These are the 5 most famous plaques:
Sherlock Holmes ~ Fictional British consulting detective @ 221b Baker Street
Charles Dickens (1812 - 1870) ~ Novelist @ 48 Doughty Street
Karl Marx (1818 - 1883) ~ German social, economic and political theorist @ 28 Dean Street
John Logie Baird (1888 - 1946) ~ British (Scottish) engineer who invented television (TV) @ 22 Frith Street
John Fitzgerald Kennedy (1917 - 1963) ~ 35th American president @ 14 Princes Gate
UPDATE April 2006: I was at Kew Gardens this weekend (Easter Sunday), and saw a blue plaque! It was for Camille Pissarro, the French Impressionist artist, who lived at this address in 1892. This plaque is found in Gloucester Street, Kew.
http://www.blueplaque.com/


