I ran across an article in PC Gaming magazine about these mini-sized monitors that can be used for a variety of things. It could be used as a message center where you keep track of social networks likes Twitter, or where you keep an eye on other info while you are playing a game on your main monitor.
It's an interesting idea, but for the same price you can easily grab a 19"-22" monitor, so what's the appeal? Well it is connected via USB only, so if you don't have another graphics port, that is certainly an advantage. If you don't have room for another full-size monitor, that could also make this useful.
It's hard to say without actually using one, but I was curious if anyone here uses these.