iCal Events widget for Mac OS X Dashboard
This widget was rated well by Macworld (http://www.macworld.com/article/1045367/widgetsoftheweek004.html) and has probably received millions of downloads, though I haven't tracked it. I continue to get emails from users who are excited about the widget, and I am open-sourcing it in hopes that others will take it over and continue to maintain it.
The source includes a full iCalendar parser written in Objective-C, including most recurrence rules (RRULE). Leopard and later systems use the Apple CalendarStore framework instead. I wrote much of this in 2005 as a college freshman, so some of the code may be very bad.