A minimal custom metronome that plays a click the HTML5 audio synthesis API.
You can use it to skip beats and only play every 2nd, 3rd, 4th beat (up to every 8th beat). This can be helpful for training your musical pulse to be steadier -- see this video by Dr. Molly Gebrian for an explanation of why normal metronome use (or disuse) isn't very helpful, and why dropping out beats is a good exercise.