A versatile, affordable, free hardware, stereo mixer for Eurorack
Each input channel has either mono or stereo inputs, a balance control, a mute switch, and a volume control. The output stage has a stereo TRS (headphone) output, but patching into the right input will break the normalling, allowing you to output stereo over two mono cables. The output also has a mute switch and volume control, so you can easily mute your system or adjust the master volume depending on where your output is going.
Each input channel is passive, so can be made very affordably, although ±12V and Earth are available from the bus which carries the stereo signals, so future/user-developed input stages could be active, for example an input stage with an active EQ.
There is also of course the potential for alternative output stages, such as balanced 6.35mm or separate output and monitor outputs for headphone monitoring during a live set.