NagThing

An app for windows, designed to help you avoid getting repetitive strain injury, or a similar ailment. At regular intervals it pops up an annoying dialog demanding that you take a break:

It runs as a taskbar app. Click the taskbar icon (a mug of tea) to bring up the settings dialog:

Download NagThing.exe

Download the C++ source code: nagthing.zip. It's MFC. ugh.