Raspberry Pi

"Raspberry Pi" is a name of a family of small computer boards for the hobbyists.

In the family are the pure computation modules and the whole computers like Pi 400.

Clones

The whole RPi idea is quite popular. So, there are many "clones" like Orange Pi and Banana Pi and also pretty distinct boards like Clockwork Pi.

Your own cluster of RPies

TuringPi, a x8 Computation Modules on single "motherboard" in ATX form-factor.

Resources

Diet Pi, an opinionated Linux distribution for the SBCs (Single-Board Computers)
"How to Use Raspberry Pi for a Professional Solution" (TLDR: Don't)
"How to use your Raspberry Pi as a wireless access point"

Pi-hosted software

DIY-IPMI, "A DIY IPMI / IP KVM system utilizing the Raspberry Pi"
Lakka, "a lightweight Linux distribution that transforms a small computer into a full blown retrogaming console."