AntiRSI is a program for OS X that helps prevent RSI (repetitive strain injury) and other computer related stress. It does so by forcing you to take regular breaks, yet without getting in the way. It also detects natural breaks so it won't force too many breaks on you. This is an evolved version from the still free 2.1 version (available in Related Links).
Please Note: AntiRSI is a reminder tool that can be helpful, but it is not a medical device. If you experience problems, please consult a doctor.