Euclid and Diophantes(?)

http://www.math.mtu.edu/mathlab/COURSES/holt/dnt/ describes how to apply Euclid's algorithm to Diophantine equations. Something from CSMN 614 (Algorithms and Data Structures) at Univerisity of Maryland.

Computing dates in your head

http://rudy.ca/doomsday.html is an interesting site that teaches you how to figure out which day of the week of a particular date falls on... using the calendar in your head. You'll also be able to calculate the date of, say, the third Friday in March or the second Wednesday in June.