Since building standing desks seem to be all the rage, I decided to put one together for work. I used an Ikea Fredrik as the base, but only used the two shelf pieces and discarded the main desk piece as I wanted it to fit in a corner. The Fredrik has multiple slots along the side allowing you to position the shelves wherever you'd like. One thing I would caution is decide once where you want the stuff as pulling it apart and reassembling is a bit of a chore and you have to built it from the bottom up. It's got quite a nice solution for cable routing and hiding power strips. I also used the Fredrik Accessory Pack mostly so I could add cup holders to the desk, but the hooks came in quite handy for wire routing and for hanging my backpack. The official instructions actually have variations which seem appropriate for standing desks. One thing I did notice with the way mine is configured is it a bit shaky if I type too hard. The vibration isn't pronounced, but its enough to slightly shake the LCD on my laptop.
I attached a 3M keyboard tray to the top shelf, but had to use some nylon spacers to attach it since there is a metal support beam running down the center of the shelf. I used two cheap LCD arms for the extra monitors -- although these tend to tilt slightly with the way I have them mounted. They also need some wood blocks added under the shelf so they have the right thickness to attach to. To top it off, I bought a cheap drafting chair for days when I feel a bit tired of standing -- but I went for one that looked uncomfortable so that I wouldn't be tempted to sit all that often.
I've been using it for a couple of weeks now and seem to be able to stand most of the day while being productive.