You can use WebDAV for calendars, files, tasks and contacts.
| Name | OS | Description |
|---|---|---|
| DAVx5 (formerly DAVDroid) | Android | Synchronise Tasks, Calendars and Contacts |
| Mozilla Thunderbid | Windows, macOS, Linux | Mail client with integrated Lightning extension for Calendar, needs plugins from Sogo.nu for CalDAV/CardDAV |