OpenHab (Openhabian actually, so some additional services like Zigbee2MQTT or Grafana)
HP EliteDesk 800 G2 i5-6500T, 8GiB RAM - this one is currently the mainstay of my lab, running containers with docker-compose
Nginx as reverse proxy (+ fail2ban)
Paperless-ngx (+ Redis, Tika, Gotenberg)
Jellyfin
Minecraft server (+ Mapcrafter)
ddclient
Heimdall
Dell OptiPlex 7060 Micro i7-8700T 32GiB RAM
I've gotten this one fairly recently. A real bargain - costed as much as the CPU alone and was in pristine condition. I will be migrating the workload from EliteDesk to this one. I decided to try ProxMox this time though, so I need to learn a bit first. Also perhaps add a second SSD