I work with scanners for a living. Do you have specific questions about them?
The type of scanners that I think your looking for fall into a category known as part scanners. They are used for quality and reverse engineering. Accuracy varies wildly and is rather complex in practice but in general they range from 0.0004” to 0.01"
I'm assuming that due to where you are asking the question you're looking to reverse engineer parts. Unfortunately unless you have a substantial budget scanning might not be right for you yet. For reverse engineering we use $50k scanning arms and up. It's not all bad news though!
If your willing to supplement your scanning with manual measurements and have a functional equivalent part as opposed to a complete copy you have lots of low cost (some even free) options.
Your options are going to be in the camera based scanners. As CAD321 mentioned the Kinect can do an alright job but may struggle with small parts. I haven't used Reconstruct Me but have used a free option from Faro called
Scenect.
Another option is Autodesks
123D Catch that will turn normal photos into 3D meshes. There is also a new offering from Autodesk that looks promising called
Project Memento that's worth checking out.