Mechanical Calipers

While this isn't the greatest looking device out there or as technologically innovative as others posted here, it certainly has to be one of the most useful. It is plain, simple and does what it needs to extremely well. I'm sure most of us had no idea how versatile they were before our workshop classes.  

It can measure diameter. depth and length. There is even a simple locking nut which allows the caliper to stay at a certain length. It does all this easily and can even measure down to increments of .2 of a millimetre.