update 04/05/16 : I relabeled some of the spreadsheet cells to be less confusing- the cells on the first sheet now match the diagram on the second.

If your axes are misaligned, you will not be able to print gears that mesh, tight fitting lids, etc.  This cube will help you align your printer's axes by checking the angles between them.

Calibrate your extruder, then print this cube (try 250 um for all layers, 20% infill, 2 perimeters).  Measure diagonals and sides of each plane and enter values into spreadsheet to calculate error and required correction in printer axis alignment.

If diagonals are equal measure, then the axes are accurately set to a 90 degree angle.  You must check XY, XZ, and YZ planes to verify orthogonality of axes.

This cube can also be used to calibrate steps/mm for all three axes.  I recommend you perform extruder and steps/mm calibration as described here: http://reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide

Note:  calibrate the extruder at the thinnest layer setting you ever intend to use.