At first, I was very sceptical about them ("Connect a screen to a USB port? That'll never work!") but I've bought one and it does indeed give you a second screen with which to work.
The small black box that you can see with the blue light in the photo below is made by EVGA and the model is called "UV Plus+ UV-16" (snappy, eh?). There are other makes/models, but this is the one that I've bought. It'll drive a monitor up to a 1600x1200 resolution.
Here is the setup with the cables tidied away... (iMovie gets a screen to itself on the left-hand screen, while a tutorial PDF teaching me how to use iMovie can be stretched to occupy all of the right-hand screen)
Okay, if you want to know anymore, just ask!