Here you can view monthly Jewish calendar, convert dates between Jewish and Gregorian (or Julian) calendars, look up candlelighting times for Shabbat and Jewish holidays and other relevant daily schedules. (Candle lighting times are currently only available to US locations, by postal code).
Update 2008-11-18: By popular demand, this site is back up as it was before the hardware problem. I thought there are many new alternatives nowadays, and was was surprised to hear that some users still like it as it has been for the past 10 years or so. I hope to find some time to make a few improvements to this remnant of last century.