BreakTimer
BreakTimer is a free and open source cross-platform program to remind you to take breaks, save yourself from RSI and eye strain
RSIBreak
RSIBreak is a free and open source software that periodically reminds users to take a break to prevent repetitive strain injuries (RSI)
Safe Eyes
Safe Eyes is a free and open source program for Linux users to protect against eye strain
Pomodoro
Pomodoro Indicator – free and open source simple software, an indicator that reminds you to take a break
Workrave
Workrave – a program to remind you to take a break and perform physical exercises to maintain your health