Wikipedia: Leap Year
Article describing the complex process in which a leap year is calculated using the Gregorian, Julian, Chinese, Hebrew, Hindu, and Iranian calendars.
timeanddate.com: Why Leap Years?
Explains why leap years are used in modern calendars, the history behind the day, and how to calculate a leap year.
Brief discussion on the reason for leap years, including graphs. From Eric Weisstein's World of Astronomy.
Why Do We Have Leap Years?
Explanation of the occurrence from Cecil Adams' The Straight Dope.