I use my RPi4 as a small server having multiple docker containers behind a Traefik proxy. The largest applications are probably my Nextcloud and my Photoprism instance.

The Photoprism instance holds about 114.000 pictures which I took over the past 10+ years and includes features like face recognition and browsing by location. I quite like it and find it super impressive that such a small device can run such a powerful application with that amount of data.