Office Chair Exercises
This video has some good isometric exercises that can help keep the blood flowing and burn some calories while you're sitting in your chair.
This video focuses on stretching your muscles while you are sitting in your chair.
This video has some great excessive for sitting in a cubicle or office chair. Although, unless you have a private office, it might be difficult, since many of the exercises are done while standing and it might be awkward to do in front of your work mates.