I have several older machines set up as servers. I've actually got a big walk-in closet with a standing AC unit with the exhaust and condensation line running out through the wall into the adjoining laundry room to utilize the exhaust port for my dryer and the floor drain for the washer to keep the closet at a steady 64° F all year.
Machines used as servers:
Votan - A8 laptop. 16GB DDR3, 2x4TB 7200RPM HDD In 4TB RAID1 seedbox/plex server
Heimdall - Raspberry pi 2B, 1GB RAM, 64GB sd, VPN server, caching DNS, and reverse proxy for http(s)
Balder - Dell Inspiron 531S Athlon X2 5000+, 3GB DDR2, 360GB HDD, httpd for pastebin, pomf clone and a web forum
Bragi - eMachines T6542, Athlon 3500+, 1GB DDR2, 160GB HDD, failover for Balder
Fenrir - hp pavilion a1700n, Athlon X2 3800+, 1GB DDR2, 160GB HDD, load balancer
Thor - hp pedestal server custom, P4 Era 2x4GHz Xeon, 64GB DDR2, 2x2TB HDD RAID1, backups for all servers except Votan, nightly scripts running with SMS notification for success/failure
Loki - Apple Mac Mini 6,2, the only one not in the closet (pun intended), C2D, 4GB DDR3, 4TB HDD, connected to TV as STB with plex client and kodi, also for browsing the internet from the couch with a Logitech wireless keyboard with built in touchpad.
Not used as server:
2014 macbook air, i5, 8GB DDR3, 256GB SSD running only Ubuntu 18.10, for browsing on the shitter and taking when I need a laptop I don't give a fuck about when I leave the house. My 'real' laptop is an i7 8750H, 32GB DDR4, 500GB SSD, 1TB HDD, GTX 1060 6GB fucking madman of a laptop that only leaves the house when I'm going somewhere I know Tyrone isn't creeping, but since it's a late 2018 machine it doesn't really qualify for this thread. Even the mba is pushing it.