Way cool. It does the basics like Julian/Gregorian, Muslim, Hebrew, etc calendars; 'puter stuff like Excel serial day number, ISO dates, etc; and esoterica like the French Republican and Mayan calanders.
You just enter a date into any calender on the page, click "calculate," and all the other calendars on the page update to the same date. For instance today, 2 January 2006, is 20 December '05 on the Julian Calendar, the 2nd of Teveth 5766 on the Jewish calendar, the 2nd of Dhu l-Hijja 1426 on the Islamic Calendar, 2.19.12.16.15 in the Mayan Long Count, 1136160000 in Unix time function, Excel serial day number 38719.