The Penny Black was the world’s first adhesive postage stamp to be used by a public postal system. The stamp dates back to Britain on May 1, 1840 when it was first issued. It was publicly available starting around May 6, 1840.
The idea of using an adhesive stamp to indicate pre-payment of postage was due to a man named Sir Rowland Hill. In 1837 he proposaled to reform the British postal system by using prepaid stamps. Prior to the reform and for a while after, it was normal to have the recipient pay for the postage upon delivery.
A Penny Black stamp in unused condition can fetch between 1-3 thousand dollars.