I've loved this little trick for years: the Doomsday Algorithm, a creation of Dr. John Conway, he who gave us the game of Life.
The Doomsday algorithm gives you the day of the week for any date, based on the last day of February.
Lots of great links at the bottom of the page, too.
Links
The Game of Life - in Java, with a link to the Wikipedia entry on the Game of Life
