The Penny Black

As part of the postal reforms initiated by Rowland Hill, the world's first adhesive postage stamp was distributed in London. With an elegant engraving of the young Queen Victoria, the Penny Black was an immediate success. The first stamps were not perforated. 

Although May 6 was the official date that the stamps became available, there are covers postmarked May 2, due to postmasters selling the stamps from May 1. A single example is known on an envelope with a postmark dated  May 1, 1840. 

"The Penny Black was in use for only a little over a year. It was found that a red cancellation was hard to see on a black background and the red ink was easy to remove, making it possible to re-use stamps after they had been cancelled. In 1841, the Treasury switched to the Penny Red and issued cancellation devices with black ink, much more effective as a cancellation and harder to remove. However, the re-use of stamps with the un-cancelled portions of two stamps to form an unused whole impression continued, and in 1864 the stars in the top corners were replaced by the check letters as they appeared in the lower corners, but in reverse order" (Wikipedia article on Penny Black, accessed 01-31-2012).