Stretchly

Stretchly is a cross-platform open source app that reminds you to take breaks when working on your computer. Stretchly itself lives in your tray, only displaying a reminder window from time to time, which contains an idea for a break. By default, there is a 20 second Mini Break every 10 minutes and a 5 minute Long Break every 30 minutes (after 2 Mini Breaks). You'll be notified 10 seconds before a Mini Break (and 30 seconds before a Long Break) so that you can prepare to pause your work.