From both a comfort and productivity standpoint, it's always a good idea to leave the computer and walk around regularly. Not only does this let you stretch, but also reduces eye strain by giving you time to focus on more distant objects than your screen. Climbing up and down a few flights of stairs is an option if the leg motion makes you feel better.
It's difficult to recommend anything else too specific without knowing what it is about sitting at the desk that causes your leg to hurt, and why only the left leg (ask a doctor). If the pain is caused by circulation or posture issues, just changing to a different type of chair or adjusting the angle or amount of cushion on the seat may help. There are also the more exotic options such as sitting on an exercise ball instead of a chair, or using a raised desk that allows you to work while standing. Both of these also make it easier to move and stretch your legs while working, if you find that helpful.