Vimcal - Vimcal is the world’s fastest calendar, beautifully designed for people who work remotely and live in their calendars.
Button for Google Calendar™ - See your daily schedule and join meetings in one-click.
Calndr.link - Dead simple & free add to calendar links + API
Calendarist - Time blocking companion for Google Calendar
Add-to-Calendar Button Generator - This open source projects lets everybody create amazing buttons, where users can add events to their calendars.
Amie - GitHub for research and data science