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
Clean Google Calendar - Makes Google Calendar look nicer & launches as a desktop app
Plan - Plan is a better way to organize.
Cal.com - Cal.com (formerly Calendso) is the open source Calendly alternative.