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.
Also Nextcloud, but Seafile for file syncing. The only reason for Nextcloud for me is the calendar webinterface.