TrueNAS Core - TrueNAS Core (formerly FreeNAS) is a storage operating system strong and robust enough to meet the needs of enterprise level businesses.
DietPi - Dietpi is a debian based operative system made to install new apps easyer.
XigmaNAS - File Sharing, OS & Utilities, and Security & Privacy
TinyCore - Simple operating system based on Linux that uses "modules", and loads everything into RAM. Can be persistent too.
Amahi - Amahi is a media, home and app server software known for its easy-to-use user interface. Amahi has the best media, backup and web apps for small networks.
FatDog64 - FatDog64 is the lightweight 64-bit multi-user Linux distribution.