This programs reminds you to take a rest from your computer every hour. When launched, it starts slowly drawing a red circle in the system notification area. It does that very slowly, not diverting any attention from what you're doing. In 1 hour it finishes the circle and starts blinking with it annoyingly, now attracting your attention to the fact that you better take a short break. After the break, reset it with the right mouse button menu, and it will start over again.
Download (~1 MB). (Windows .exe)
Sources (~1.5 KB). (C++, wxWidgets, tested on Windows XP, Windows 7 RC1 and Ubuntu, wxWidgets 2.8)
And in Python (~1.5 KB). Requires wxPython. Consumes about 20 MB of memory on my Windows machine, and about half as much on my Ubuntu machine. Perhaps, too much for such a small one-purpose thing.
You may also want to try ChronoControl program by Vladimir Kim. It's a way more serious software for, basically, the same purpose. I couldn't use it because it was policing me too much, but maybe that's what some people need?..