hoggy i've tried each of these solutions. I like to stand when I work (i also pace sometimes, i also kneel sometimes -- anything but sitting still which i hate to do for too long. I find i focus and concentrate better if i'm a bit uncomfortable actually). In addition to the below, I'd like to eventually try a standing lectern next to my desk; a treadmill (why not), and one of those
bouncy balls that people sit on at their desk (friend had one, tried it, I thought it took too much effort to sit on it, tho he swears by it), and an
ergonomic kneeling chair (never tried one but am intrigued).
I did try an
excercise disk by the way, and thought it actually tired me out after a while. My goal isnt to get tired out, lol, but to simply remain alert and be able to stretch a bit sometimes.
Sit-to-stands:
1.
fully articulated keyboard platform: Despite the mediocre reviews, this was the model I once owned, and it moves the keyboard around in an incredibly wide range, tho it takes some work to install it under your desk, it WILL allow you to stand easily (the keyboard platform can simply be pulled upward to any height).
Pricey. You can find them on ebay sometimes for about 100 bucks tho.
Note that this model is a true "sit to stand" keyboard platform; ie, its not simply a minimally adjustable one for comfort, its range of motion is HUGE.
Incidentally i also at one point owned one of those ergotron monitor arms and it was very adjustable tho I eventually went back to a simpler monitor stand when I upgraded my monitor. But the monitor arm worked well with the keyboard platform.
2. I also infamously have (and occasionally use, lol) one of these.
http://connect-a-desk.com/:-/ You should see me using it with my miner's light strapped to my head.

Truth be told its handy if you're a pacer, as I am. I dont put my laptop on it tho, just paper. I also use it to prop up books against my chest while typing at my desk.
Not a true sit-to-stand but thought i'd mention it.
3. In my dreams though, I own this:
http://www.geekdesk.com/default.asp?contentID=604I mean,
come on! This to me is the ultimate sit-to-stand solution. Whole desk rises electronically. Btw there are a number of manufacturers making these now, not just one.
Pricey as heck but worth it I think. I'll get one some day, probably two of them actually.
4. My current sit-to-stand solution is considerably cheaper but works very very well.
Inspired by this vaporware product, I convinced an ebay seller of plexiglass to create this for me. He took a long sheet of plexi and bended it for me and shaped the edges smooth. Works 100% for this purpose. I leave a keyboard plugged in on my regular keyboard tray, and above it, i put the plexi tray, which holds my second keyboard.
I leave both keyboards plugged in constantly, so if i want to stand -- i simply stand and keep typing. 
My monitor can be raised by just pulling it up with one hand, if needed, while i'm standing. Works well for the moment and is very cheap. I like the two-level keyboard effect, and its nice to simply stand and keep typing without having to adjust anything or plug anything in.