I was using IKEA Billy bookcases in my office and put 3 side by side, and wanted a way to connect them to make them more sturdy. Yes I could have gone out and bought a metal plate that was pre-drilled, but I have a 3-d printer, so I just made one.

You may note in the pictures a small amount of bending due to the legs being too long, and the screw holes not quite in the center, I did make some small adjustments to the model before uploading, but the ones I printed worked just fine for me.