Cron - Cron Calendar.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
zcal - zcal is the fastest way to schedule every meeting for Free and make it personal.
Laverna - Laverna is a JavaScript note taking application with Markdown editor and encryption support.
Cal.com - Cal.com (formerly Calendso) is the open source Calendly alternative.
doo - A smart, simple app for reminders and to-dos