When I used OS X full time, I found a nifty little tool named AntiRSI. True to its name, it prevents you from working too long on the computer by eclipsing your screen and forcing you to take a break.
Fast forward about a year, and today I found out about Workrave. It reminds you to take breaks just like AntiRSI, but improves on the idea by giving suggestions of stretches. Even better, all it took was an apt-get install workrave:)