This is my first post, so I don't have permission to edit the WIKI page yet. Created the account just to share that the Foscam W21 (https://www.amazon.com/gp/product/B088LW8MZP/ on sale for $30) USB webcam works fine with Octoprint on my OctoPi.
Note: to get around a library error, I did have to add 'export LD_LIBRARY_PATH=/home/pi/mjpg-streamer/mjpg-streamer-experimental' to /etc/profile. Further, I set camera_usb_options="r 1280x720 -f 20 -d /dev/video0" in boot/octopi.txt, but that is likely unecessary.