I've been using
this program, called Ergocise, for the last few days and I like it. It pops up a window at a user-set interval (15, 30, 45 or 60 minutes) with a stretching exercise. Each exercise includes a description and pictures. Some are done while sitting and some while standing.
It's good for those of us who spend most of the day sitting in front of a computer and need a little reminder to stretch.