Jewish Calendar (known on the Mac App Store as "Original Jewish Calendar") is a calendar that displays dates and holidays in the Hebrew Calendar. It uses romanized names for the Jewish months and holidays, which everything from Hanukah to the most obscure of occasions.
The application was ported directly from one designed for 68k-based Macs, and has retained the extraordinarily basic (and somewhat nostalgic!) black-and-white appearance of that era, despite a perfectly modern program icon. As for what the calendar displays, the Jewish date is shown under each date in the tabulated Gregorian (or, for historical work, Julian) calendar, along with any holidays. The holidays themselves can be customized to include one or more of Parsha, Omer, and Chol Hamoed, in addition to using either the dates of Israel or the Diaspora. Years and months can be browsed with "Next" and "Previous" buttons or by inputting them manually into the application's menu options.
Sam's Related Factoid: Did you know that leap years in the Gregorian calendar only occur on years that are multiples of 4, but not multiples of 100, except when multiples of 400? I didn't!