Super small, super cheap computer designed to run Linux. The board itself is about the size of a pack of cards, and cost about $80 although earlier models a few years ago before prices of everything skyrocketed was about $35. It’s a fully fledged computer and with a microssd card loaded with a Linux distribution, a keyboard, monitor and mouse (last two optional) you can have either a command line only or fully fledged gui OS up and running in minutes.

Not super powerful but it’s low energy requirements and form factor make it great for tinkerers and specific tasks/functions. Home automation, video game emulation, pi hole (network wide dns ad blocking), and home based VPN services are a few that pop into mind.