Customizable Ruler

One ruler to ring them all.
(Or something like that.)

With this you can make rulers and scales for a wide variety purposes. The most obvious being a regular 30cm/12in school ruler (school_ruler.stl is attached if you want that), but you can do pretty much anything you can imagine: LEGO units, drafting scales, etc.

Also, you can alter the counting direction and number orientation, so you can use this for all sorts of jigs and tools. See depth_gauge.stl for one example.