Do you mean implicit clock to save the third line? Other than that SPI is about as simple as it gets. Plus there's a protocol for practically every requirement:

Do you mean implicit clock to save the third line? Other than that SPI is about as simple as it gets. Plus there's a protocol for practically every requirement: i2c for simple slowish shared bus with clock, 1-wire shared bus without clock, SPI fast and simple, serial single ended (RS232) or differential (RS485) for stuff outside of your PCB...