Or the cheapest version buy a fixed standing desk, and when you want to sit use a stool, or a very tall chair.

Alternatively, put a chair (even your own chair!) on your current desk and your stuff on the chair when you want to stand.

There are sitting/standing desks that are quickly adjustable via electric motor, but they're quite overpriced so it's best to let your employer buy one for you. My officemate has one very similar to the one here:

http://www.thehumansolution.com/uplift-hover-adjustable-heig...

Try the Geekdesk [http://www.geekdesk.com/]

I got a Geekdesk and put my own top on it from Ikea. It works perfectly for me. http://jsndev.net/104/geekdesk-and-ikea-standing-desk/

Very cool! Pricy, still, but cool. Thanks for the tip!

I bought myself one off ebay for a couple of hundred pounds, has a little up/down controller which takes no more than a few seconds. I sit more than I stand, but it's a nice break. Not sure how I'd feel about this in a office though!

http://cl.ly/image/2x0R2H2q330B