For Ubuntu, try this: http://askubuntu.com/questions/63999/how-can-i-install-typin...

Here it is: https://gist.github.com/4451731

I'll post it in the morning when I'm back at work. It's just a dead-simple Tkinter app in Python that tells me to take a break. My work computer is Windows and I scheduled it using the Task Scheduler. At home I'd add it to my crontab.