Sometimes you just want to take a couple dimensions off a part, but you ONLY have access to the STL file. It's difficult to convert an STL file back into a 3D geometric object. (It can be done to some extent but I don't want to purchase and install special software to do a quick and dirty task.) In times like this I've found it useful to load the part in question into your printer host software (Repetier Host for me) and measure it with a 'digital ruler' STL file. The ruler provided has 1/4" increments, but you can scale it to measure any increment you want. In the example photos I measured the hole spread (3/4" thank you) on a Printrbot part. Obviously, this method is very limited but it IS free and straightforward. I've also included an STL file of a soda can which can be used as an alternative size reference. UPDATE: You might prefer to use this easy & free program for taking dimensions of STL files: http://www.netfabb.com/basic.php