Exercise produces endorphins. Endorphins make you happy.
Go for a run during the day, or if running is too much just take a brisk walk. Even a few loops around the office building might help you relax and refocus. And if you can't get out, find an empty conference room or corner of the office where you can perform some basic stretches. Anything to get the blood pumping so you can turn that stress into something positive.
If you're feeling a little more ambitious, join a gym near work and hit the treadmill, elliptical or yoga class during your lunch hour. A little sweat and endorphin rush could be just what you need to de-stress.