UPDATE 3 Jun 23
Added version 4 (v4) of the SCAD file. Number of improvements, including an automatic notation scribed into the bottom of the box that records the dovetail and box size (useful if you have versions with multiple sizes). Also added in STLs for 1x2x20mm and 1x1x20mm, by request.

UPDATE 17 Nov 19
Added version 3 (v3) of the SCAD file. This version adds the ability to give a rounded bottom to the box. This is useful when using for small parts storage, or like I am for drill bits (see new example picture). The latest version of OpenSCAD has a very good function to easily edit the parameters and create new boxes - highly recommended to use that with the SCAD file to customize the size of box you might need.

UPDATE
Added version 2 (v2). In this version, the bottom of the print has "anti-elephant foot". This makes it so you don't have to clean up the first layer in order to fit the boxes together. They should work straight off the printer, as long as the "fit factor" is sufficient. I've been using 0.15 lately and the boxes fit fairly well.

ORIGINAL
I thought there was something like this already on Thingiverse but I couldn't find it, so I made my own. This is a customizable, interlocking box using dovetails to connect. I'm using these to do some organization of tools and small parts.

The basic concept is to define a "unit" size of the boxes you will make, and then make the boxes in increments of those units. By defining a unit size and sticking to it, all of your boxes will then be able to connect to each other.

You can have one or two walls that are open. That way, you can connect two boxes together to create one that is longer than your print bed.