I was on the fence about whether this one was worthy of a post, but given that it has had quite a bit of attention of Thingiverse, I decided it must be worthy.
We bought a thousand CR2032 batteries for a Maker Faire event one year. I tossed them all in a big bag for storage and as I was placing them on the shelf, I noticed that the bag was on the verge of melting. Who would have thought that a bunch of flat batteries could short each other out? Now I know better.
Needless to say, it was time to find a better solution than big bag. I designed this coin cell holder entirely in OpenSCAD. It took a few iterations to get the fit just right, but that's the beauty of having a 3D printer. The source files can be found here