Userspace-accessible GPIOs, I2C, SPI, PCM, and UART on a system that runs Linux. My employer uses them for a bunch of our hardware-in-the-loop test automation, with the GPIOs used for CAN, relays for switching various signals, vibration table control, etc. The USB gets used for SCPI device control (power supply, multimeter, etc.) and DuT connection. It's a lot cheaper to use a Pi for this than it is to use a small form factor x86 machine with a bunch of USB-<protocol> dongles.