gcalcli is a Python application that allows you to access your Google Calendar(s) from a command line. It's easy to get your agenda, search for events, add new events, delete events, edit events, see recently updated events, and even import those annoying ICS/vCal invites from Microsoft Exchange and/or other sources. Additionally, gcalcli can be used as a reminder service and execute any application you want when an event is coming up.

gcalcli uses the Google Calendar API version 3.

Found on https://www.raspberrypi.com/news/pico-powered-progress-bar/

GriCal (which is a short form for Grid Calendar) is an open web where people store quickly a few information about interesting events, so that others can also easily know about them. GriCal also makes easy and fast to get notified and store interesting events in its own external calendars.

DavMail is a POP/IMAP/SMTP/Caldav/Carddav/LDAP exchange gateway allowing users to use any mail/calendar client (e.g. Thunderbird with Lightning or Apple iCal) with an Exchange server, even from the internet or behind a firewall through Outlook Web Access. DavMail now includes an LDAP gateway to Exchange global address book and user personal contacts to allow recipient address completion in mail compose window and full calendar support with attendees free/busy display.

Comparison of the Emacs packages Org and Planner.