Most of them make things out of ABS - lightweight stuff like black plastic drain pipes are made from. However, apparently, most of them can also be made to "print" things out of cake frosting. The cheap ones that you can afford are limited to objects about the size of a coffee mug. And the software isn't exactly plug & play. You have to model the object in a 3D design program, translate the file into a machine-readable format, then translate the file into a printer instruction file.
But people are improving them all the time.http://www.makerbot.com/