Eurorack

Germany's Doepfer launched its A-100 system in the mid-1990s, borrowing dimensions from industrial 19-inch racks. Width is counted in HP of 5.08 mm, power is ±12 volts over a ribbon cable, and the signals between modules are control voltages on 3.5 mm leads. Because Doepfer published the spec openly, anyone could build modules — and everyone did. There are now thousands of them from hundreds of makers.

Each module does one thing — an oscillator, a filter, an ADSR, an LFO — and the sound comes from how you patch them. The hobby's well-documented side effect is that the rack never gets full, only more expensive.