This is my most useful Eurorack module: A breadboard protyping module.

Features

• Half-size solderless breadboard for building prototype circuits
• Power buses along the sides of the breadboard (+12 V, -12 V, ground)
• Two jacks to patch signals in and out
• Socket header for connecting to the jacks and the Eurorack system bus

I have not published a detailed design for this module because it’s simple to make and, honestly, it’s pretty janky.