My friend Jenblossom reminded me to go back and check for this piece of software, i was waiting for the OS X version.
A friend at work had a consultation with an ergonomic specialist on the faculty who designed this cool software to remind you to stretch. (It is for people with RSI problems.) You can set it for how frequently you want to be reminded. It also works as a good way to notice that you have been on-line too long.
It is called Stretch Break and is $45 commercially, with a free 10-day trial:
But there is also a kids version that pretty much does the same thing for free:
http://www.paratec.com/sbform/kidsform.htm
There is a pretty good description there.