The Apple Magic Trackpads are great because you can add it to any existing computer setup. It is my recommendation for a multitouch trackpad.
Multitouch touchscreens are a different story. Unfortunately, I wouldn't "recommend" any touchscreens you can find on the market today on traditional laptops or monitors. I have heard of monitors with a newer eGalax touchscreen that supports five touches, but I haven't played with one to be sure. The best touchscreens are actually found on Android tablets. They tend to have the Atmel maXTouch chip, which recognizes and tracks many simultaneous touches. However, you have to figure out how to get a Linux desktop running on them first, and they aren't really designed for that.
I personally develop with a laptop with an N-Trig touchscreen. It does a passable job for development purposes, but it often drops touches or emits touches that don't exist.