Workrave - Workrave is a program that assists in the recovery and prevention of Repetitive Strain Injury (RSI).
Cron - Cron Calendar.
SmartBreak - Apps and tools from InchWest are designed with one goal - to increase your productivity and improve your experience on MacOS, Windows and iOS devices.
zcal - zcal is the fastest way to schedule every meeting for Free and make it personal.
StretchMinder - Work break timer with guided movement & breathing exercises
Cal.com - Cal.com (formerly Calendso) is the open source Calendly alternative.