I use a sitting desk because . . . actually, I'm not sure why it's necessary to argue for sitting down while working. Nothing is better than a good sit, except possibly a good lie down. My sitting desk is definitely a considerable improvement over Alex's original office design.
Other people, though, prefer to keep moving constantly. They may use treadmill desks or running gazelle desks. Now there's a new option: this desk built by Robb Godshaw. His hamster wheel desk gives you the feeling of moving quickly but going nowhere.
P.S. I have yet to acquire the time or the money to build my proposed mechanical bull desk. Attention venture capitalists: talk to me if you're interested.