The answers all depend on complexity.
For simple objects some calipers or a ruler along with Fusion is the best way to go. Complex organic shapes will be better to go with a 3D scan.
3D scanning is something I haven't tried yet. There are some programs that exist to work to stitch photos together, some might even be web apps. After creating a mesh there is some clean up work to be done. Scorpion Technologies have a sweet scanner set up.
Of course the easiest thing of all is to check Thingiverse and see if someone hasn't already gone and modeled and uploaded it!