There can’t be that many servers left that aren’t powered by some model of Raspberry Pi. The latest, from Michael Klements on the DIY Life website, is a 3D printed and laser-cut to look awesome, comes with a UPS to survive power failures and an OLED screen for vital information too.
The guts of the server is a Raspberry Pi 4 with a 32gb Micro SD card and cooled by a low-profile Ice Tower fan and heatsink. The UPS is via a module from GeeekPi via AliExpress, and is filled with a pair of Kiminors 18650 3.7V 2600mAh Li-ion rechargeable batteries. The UPS acts as a HAT on the Pi, but actually fits beneath it, making contact with the bottom of the GPIO pins via some spring-loaded pogo pin contacts. Klements was able to run the box for an hour and a half from a fully charged set of batteries.
The OLED display has a resolution of 128 x 64 px and is attached to more GPIO pins and communicates through the I2C interface. Information, such as the status of the UPS batteries, IP address and CPU is displayed on the OLED. The whole project is powered using the Pi’s USB-C connector.
Apart from running Raspberry Pi OS from the Micro SD card, Klements doesn’t make mention of any specialist server software (though there are scripts for automatic shutdown should power become a problem), and nor does there appear to be storage beyond 32GB on the device - presumably these are added depending on your specific use case, and the two USB 3 ports on the Pi (unfortunately front-mounted in this build, so constantly visible) make good places to attach external hard drives.