The first Christmas card was posted in 1844. The sender was W.C.T. Dobson of London's Royal Academy. The practice rapidly became popular, and a great fan was Queen Victoria, who one year, it is said, sent some 2,541 cards.
What made it especially nice for the Queen was that all the stamps on the envelopes carried her portrait: the famous one penny black, the first British postage stamp, introduced in 1840.
If you turn up one of those stamps today, you'll find many collectors eager to give you $3,000 U.S. for it. If you also have a card signed by Victoria and Albert, you can probably count on a few dollars more.