These cable tags were designed to easily differentiate self made cable harnesses in my printer mod project. I have printed them in different colors and multicolored, but you could also use a sharpie to write something (e.g. "X,Y,Z,E") on the side. They have an outer diameter of 8mm and can easily be added to and removed from the cables.

The ones shown in the pictures are the 8mm version.

Updates: 2019-03-29: Added 6mm, 7mm, 9mm, 10mm versions and narrowed the slot for better hold.