Nextcloud

shared scheduling on your existing Nextcloud

Overview
Company	Nextcloud
URL	nextcloud.com/calendar
Pricing	free— Free -- it's an app on top of the Nextcloud instance you already run
License	open
Hosting	self

CalDAV-based shared calendaring app bundled with Nextcloud: team calendars, event invites, resource booking. Zero extra infrastructure if your org already runs Nextcloud for Office Suite (see that entry for full pricing/hosting details) -- this is just enabling the calendar app on the same instance.

Usage Notes

Syncs with any CalDAV-compatible client (Thunderbird, most phone calendar apps) so people aren't locked into a web UI.