One of the worst parts about sitting at a desk for 8 or more hours a day is the fluid that accumulates in the lower legs. This can make your legs appear swollen and, in my experience, feel kind of funky and tight.
A foot pedaler, desk stepper, compact desk exercise bike or desk elliptical can encourage blood flow and prevent fluid buildup throughout the day. This pedal exerciser is inexpensive and has good reviews. You can fine-tune the speed level and intensity of your workout, and the textured pedals ensure your feet won't sleep off the desk cycle.
If you want a more substantial workout than just a food pedaler or desk bike for desk exercise, check out this under the desk elliptical. Compression socks can also help with leg swelling.