DeskTimer
I've been known to get so wrapped up in my work that I forget to get up and move around, which apparently isn't very healthy. So many years ago I knocked this together in VB6 to remind me every n minutes to at least shift my backside a bit. Thanks to Microsoft's attention to backward compatibility, it still works, and I'm still using it.

It shows a countdown icon in the notification area, because an actual window would either get hidden behind things or get in the way of things.
It counts down minutes in green:

then seconds in orange:

then flashes an annoying yellow 00 until you click to reset it:

Right-clicking opens a simple configuration window to change the countdown period.
Known Issues
- The program assumes that the Tahoma font is available.
- If the system DPI setting is non-default, digits may be drawn at the wrong size.
- Could do with rewriting in something more modern.
Where To Find It
Source code and .exe are on GitHub
If the .exe complains about missing comctl32.ocx, get it from Microsoft via this trickery.