For synchronizing address books and calendars on my Android smartphone I’v been using DAVDroid for quite a while. It is Open Source and also available on F-Droid. However it is now called DAVx5 – why?
The reasons are explained in this tweet – using “droid” as part of a name violates a trademark of Lucasfilm. Also see this extensive article at Priceonomics. Therefore the creator of DAVDroid decided to change the name to “DAVxxxxx” and replace “xxxxx” by “x5” – even though the official statment says something different ;-).