Calendar Lock PEA - Calendar Lock PEA is an encrypted desktop calendar with a day, week and month view. The shown calendars are never stored unencrypted on your disk, but exist only in the RAM. Platform-independent, Open Source, no installation or registration required.
Cron - Cron Calendar.
Simple Calendar - A simple calendar with events (optional recurring and reminders), week numbers and a customizable...
zcal - zcal is the fastest way to schedule every meeting for Free and make it personal.
UKs Kalender - This program helps you organizing your dates and tasks and reminds you of upcoming events.
Cal.com - Cal.com (formerly Calendso) is the open source Calendly alternative.