An 8 inch mini monitor.
I find myself . . . using it for everyyyyythhiiinnnng. Videos, documentation, Remote Desktop Sessions etc etc etc. It even has composite and BNC so I could hook up a playstation if I wanted to. :)
I bought it just because the thought of having it was funny. And holy crap the size difference!!
I have 2 monitors and sometimes I wish I had a tiny monitor so I can put any video calls in the corner so I don't have a dumb overlay on my productivity monitors
To add to this, I use an old iPad as a third monitor at work for Teams mostly. It's 1024x768 which seems to be the minimum supported resolution for Teams (except for some applets) and I don't need a good quality picture for Teams.
Unfortunately the app I've been using (TwomonUSB) seems to be no longer available for Windows download (CDN down), but this doesn't affect me personally as I still have the setup anyway.