I use 3D scanners for this; it's a lot easier than modeling by hand for complex objects. But if you're trying to replicate a mechanical type of part, you usually have to "reverse-engineer" the critical features, using the digital calipers or whatever to retrieve enough info to create an accurate CAD model.

If your parts aren't mechanical, and you can't afford a 3D scanner (although there are some relatively inexpensive ones now), you might look at photogrammetric solutions, which reconstruct parts from different 2D images. Autodesk has a free photogrammetry application called 123D Catch which you might find interesting.

Andrew Werby
www.computersculpture.com