I have an adjustable keyboard tray from Ergonomic Concepts (ECI-650 SST $258.00) that the employee health and safety folks like to recommend. It's OK, though I'd prefer to have a work surface that goes up and down as well as the keyboard/monitors. This tray can be modified to fit most keyboards (I have one of those microsoft Natural Keyboards -- the split kind). The surface can be tilted easily, and it moves very smoothly from the down to the up position. You can also swivel it from side to side, though I am not sure why I'd want to do that. My monitors are on adjustable poles, and I can easily move them up and down and tilt as needed for my very frequent position changes.
When seated, I am on a balance ball that I liberated from the fitness center after a dramatic sledding incident last winter that left me unable to use a chair for a few weeks. I went down and paid them for it when I decided I never wanted to sit on an office chair again. And that was before I read the sitting will kill you study! (OT: Also, I have narcolepsy, so this is a great arrangement for me. And Zev I am totally going to try your push-ups and stair sprints idea.)
Before I got this tray, I simply used a copypaper box placed on the desk with the keyboard on it, and a mousepad trimmed to fit on a boy scout fundraiser popcorn tin that has been kicking around the office for years. Not really pretty nor easy to switch from one to the other, but free, and a good way to decide whether you want to go ahead and drop some change.
I would totally go for the geekdesk if I could, though.