There was a useful article in our local community newspaper in January with some stretching exercises.  Here are a couple that you can do at your desk:

Do leg extensions
Grab the seat of your chair to brace yourself.  Extend your legs straight out in front of you so they are parallel to the floor.  Flex and point your toes five times.  Release.  Repeat.

Look up to release upper body
Sit up tall in your chair (sitting) or stand up (standing).
Stretch your arms overhead and interlock your fingers.  Turn your palms to the ceiling as you lift your chin up, tilt your head back and gaze up at the ceiling.  Inhale, exhale, release.