There isn't only one place online to go for this, however, Calendarhome is my calendar converter of choice. Genealogists are most likely to come across the need for one if they are converting between the Gregorian and Julian calendars, or between the Gregorian and a specialized calendar, such as the Hebrew, Islamic, or Bahá'í
I like this converter as it is compact. You enter one date, and all the other dates change with it. It also allows you to enter the time of day, which is important for converting between calendars where the day starts at different times.
For example, the image above shows that April 30, 2010 at 8 pm is equivalent to Iyyar 17 5770 on the Hebrew calendar. However, if I entered 4 pm, it would say Iyyar 16, since the Hebrew calendar advances at sunset.