I'm using an RPi as the main compute module for an Autonomous Radio Controlled Car. It's similar to a Donkey Car, but not using ROS (at the moment). Project is documented here: https://medium.com/@mikkelwilson/autonomous-rc-car-part-0-6e... (This is not a commercial project; just for fun)

I'm very interested to see how OpenCV 4 and YOLOv3 object detection will run on the new RPi. At the current trajectory I will have to upgrade to a Jetson Nano to get hardware acceleration (CUDA), but resorting to specialty hardware seems like cheating.

They also make a great prototyping platform for IoT projects. I've built 802.11.4 (Zigbee) mesh radio networks for passing small messages across neighborhood distances.

Previously I used an RPi to run an Airplay Bridge to my Sonos speakers. This has since been supplanted by AirPlay 2, which Sonos supports.