1-Wire
A Dallas Semiconductor bus that gets by with a single data line plus ground — and can power its devices over that same line.
The line is held high by a pull-up resistor and pulled low by whoever is talking. Bits are encoded as short or long low pulses inside time slots the master starts. Every device has a unique 64-bit ROM ID burned in at the factory, with a family code and a CRC, and a search algorithm lets the master enumerate everything on the bus without prior configuration. In parasitic-power mode a device charges a capacitor while the line is high and lives off it while it is low.
Speed is modest, around 16 kbit/s, but a DS18B20 temperature sensor at the end of ten metres of cable does not care. The same technology sits in iButton, the key fobs that still open doors and laundry rooms. Its tight timing means it is often bit-banged with interrupts disabled.