DAVdroid integrates CalDAV/CardDAV into Android! GPLv3, available in various stores.
More than 20,000 licenses sold. Winner of the Constantinus Award 2014 (category: open-source).
ICSdroid synchronizes external calendars (.ics/iCalendar) with your Android device in regular intervals.
Typical use case: school/university/club calendar
Community related to the free image manipulation software GIMP, featuring tutorials, news, forums, polls and contests.