My 3x3 matrix mixer is a relatively boring, but flexible and useful module.
It has 3 inputs on the left, in a column. It has 3 outputs on the top, in a row. Below each output is a on/off/on mini toggle that selects a normal output, an inverted output, or turns off the output.
Below each switch are 3 attenuators, 1 for each input. At the very bottom is a row of dc offset controls. The dc offset controls are push/pull types, and must be pulled out to add dc to the mixer. They are internally calibrated for +/-5V, with zero in the center (this can be internally changed to a min of 3.3V or a max of 6.3V).
I typically take advantage of the ability to input a single CV, and then output that CV and an inverted version to two separate destinations. Taking a ADSR envelope and controlling two bandpass filters this way can get great vowel sounds. I can also patch up a through zero flange effect with my dual delay unit using this mixer.
One improvement I plan to make is to have both the normal and inverted outputs available via separate jacks, instead of the single output with switch scheme I have now. Often I am using two outputs simply to access both normal and inverted outputs of a single input simultaneously.