I use CalDav for calendar, contacts and tasks sync and share with the rest of the family.
Server is Nextcloud with WebDav/WebCal/CardDav and DavX5, Etar calendar, native contacts and OpenTasks on android. On Linux WebDav to access and sync Nextcloud files.
How do you use them?
Edit: Added CardDav for contacts.
I have my groupware PIM server hosted by Kolab Now and mostly use KDE Kontact to access the functionality on PC/laptop and the default apps that come with Blackberry 10 OS on my old smartphone.