what is a good speed measuring device, that i can use while kite skiing on a frozen lake?

As far as i know there are two main options people will use.
You can get a specific gps tracker / speedo (garmin etrex is a popular one). Or you could use an app if you have smart phone (runkeeper/ endomondo are ios examples).
A specific gps is prefered, but more expensive.

I'm sure other will have more ideas, i'm not as knowledgeable as many on the forum.

Hope this helps,
Tim

Garmin E-trex are popular, relatively inexpensive, and easily fit in your pocket. I set mine to track maximum as well as current speed, so that if I cannot look at it every moment in the buggy, I know my fastest speed.

Im itching to get a garmin fenix Watch. It will be harder to lose than my phone was. Note to self, use zipped zipper pockets.

The newer model etrex 10 are good. They use a mini USB connection, so easy to connect to your computer. The older etrex H that many buggyers use require a special cable.

For skiing you might want to look at recon goggles
http://www.reconinstruments.com/products/snow-heads-up-displ...
That way you won't be carrying a hard object around with you, and you can see the info on the go.

I just looked at a couple of vids on those goggles, they look awesome

I use the Recon system and it's great. The goggle itself is awesome but not having to use my hands to see my GPS is priceless.

if you are looking to measure speed in a buggy, you may be able to use one of those computers for a road bike, i put one on my bike for 30 bucks, now I am not sure if the computer it self has taken into account the tire size that might only be used the track distance travelled, but it might work for speed

Bike computers are calibrated either to a set size or to the measured circumfrence of the wheel with tire.

It also takes into account tire width.