For my daily work as a robotics engineer I was in need to visualize frames and transformations relative to each other. This frame contains screwable X/Y/Z axes and comes in two different sizes.

In addition, each cubic base features an 10mm hole on each side of the adjacent axes so that you can glue in 10x3mm magnets. This way, you can also attach the frame to joints or flanges of real robot arms so that you can observer their movement.