Clock Web App is an Open Source project that brings a feature rich Clock app to any system for completely free. It is developed for Web Based Operating Systems and is powerful software that is easy to use and easy to develop as it has a clean code base. Clone the repo, serve the public/ subdirectory, and that's it. No build process.

A CLI stopwatch utility written as a self-contained shell script. Counts up, can save the last time value and resume at that point.

A countdown timer/stopwatch that runs in your terminal with nice, big ASCII numbers.

A self-hosted service that pings webhooks or other URLs on a user-defined schedule. Works a little bit like cron. Can even do things every X minutes or hours, like cron.

Written in PHP, uses SQLite.

If you don't want to set it up yourself: https://hookless.co/

A website that takes a parrticular frequency (in KHz) and a duty cycle (at least 50%, or on half the time, off half the time) and calculates the values of the two resistors and the capacitor required to generate that frequency using a 555 IC.