While working on a model of a toy truck, I needed a module to make cubes with arbitrary edges rounded. Looking around, I couldn't find anything that quite did what I was looking for. Sembiance's OpenSCAD Rounded Cube Corners came closest, but I don't like the intersection of three cylinders look when three rounded edges come together.

I'd been playing around some with hulls around corner spheres already and when looking for inspiration, came across agodinhost's OpenSCAD Rounded Cube doing just that.

Combining the two ideas together, this library uses cubes, cylinders, and spheres to define the corners based on which edges should be rounded, then draws a hull around the whole thing to make the cube. This makes for very nice looking corners and edges for every arrangement I can think of.

Calculating hulls around curved primitives isn't the fastest operation, so I generally do my drafting with $fn around 30 or lower, then crank it up to 100 or so for STL renders.