Raspberry Pi - The Raspberry Pi is a tiny and affordable computer that you can use to learn programming through fun, practical projects. Join the global Raspberry Pi community.
DietPi - Dietpi is a debian based operative system made to install new apps easyer.
Orange Pi - It’s an open-source single-board computer. It can run Android 4.
TinyCore - Simple operating system based on Linux that uses "modules", and loads everything into RAM. Can be persistent too.
Odroid - The Odroid is a series of single-board computers and tablet computers created by Hardkernel Co.
DebianDog - DebianDog is very small Debian Live CD shaped to look and act like Puppy Linux.