The aim of the software packages listed below is to remind you to take regular breaks from typing at your computer. The frequency and length of typing breaks can be configured manually; in most cases it is also possible to configure the software to actually lock the keyboard to prevent constant typing.
Xwrits for Unix/Linux - Xwrits manual
MacBreakZ for Mac (N.B. Shareware not freeware!)
Shareware/low-cost software (Windows and Macintosh)