There are loads though
Theres the DOD fx25, Qtron, Chunk agent 00funk, moog low pass filter (those are my favorites besides the mu-tron)
and whichever multi-unit you could get, the envelope sounds will suck compared to the stompboxes.
And yes, higgie is selling a mu-tron that I would have bought if I had £350. Good price I reckon, with the power supply an all!