Passive Utility Box
A few basic circuits-- two four-way multiples, two 1/4" <--> 1/8" converters, two attenuators-- in a handy little box that will never need to be plugged in or have its batteries replaced. Plus, it's got the form factor of an ARP MSL module! Hooray!
If I had only drilled the converter section a hair closer to the mults, I could have fit another pair of them in there! Oh well. There's always next time.
That said, next passive project: 3 to 1 mixer with tone controls. The always reliable Beavis Audio Research has diagrams for both in its Potentiometers section.