Cron - Cron Calendar.
Vimcal - Vimcal is the world’s fastest calendar, beautifully designed for people who work remotely and live in their calendars.
KosmoTime - The to do list with super powers
Button for Google Calendar™ - See your daily schedule and join meetings in one-click.
Amie - GitHub for research and data science
Cal.com - Cal.com (formerly Calendso) is the open source Calendly alternative.