Alternative OS

Lakka is the official Linux distribution of RetroArch and the libretro ecosystem. Each game system is implemented as a libretro core, while the frontend RetroArch takes care of inputs and display.

Mobile-optimized with the flexibility, ubiquity and stability of the Linux core and a cutting edge user experience built with the renowned Qt™ platform.

Recalbox is a light embedded system created to run on the raspberryPi ,raspberryPI 2 and on PC x86/x64.

You can transform your rpi or your pc in an emulation platform, supporting up to 40 systems !

Features

Supports Amiga, Atari 2600/7800, C64, NES, Game Boy, Game Boy Advance, DS, Super Nintendo, DOSBox and ScummVM, Master System, Megadrive (Genesis), FBA, iMame4all, PCEngine, MSX1/2, PSX, SegaCD, Sega 32x, and more!
Build with buildroot, so the root file system is only 100MB compressed.
Rescue system based on NOOBS : reinstall directly from your sd card or get the last version from the net
Online update
Network access to rom folder, screenshots, saves.
fat32 rom partition : compatible with all systems
Controller configuration in the frontend : configure once, play everywhere.
Background frontend music
PS3 and Shanwan Bluetooth built-in support (plug a controller, unplug and play)
French, English, Portugues (thanks to mgoulart), Spanish, German and maybe other to come if you participate.
Frontend based on Aloshi great EmulationStation2
FBA optimized version with 4 player support (yeah Dungeons and Dragons)
Use RPi GPIOs as controllers

RetroPie allows you to turn your Raspberry Pi or PC into a retro-gaming machine. It builds upon Raspbian, EmulationStation, RetroArch and many other projects to enable you to play your favourite classic games with the minimum set-up.

batocera.linux is an operating system (like Windows 10 and Mac OS) specialized in retrogaming
available for your pc and different nano computers like the Raspberry and the Odroids.
Note that you must own games you play in order to respect legality.

A free and open-source operating system for smartphones and tablet computers, based on the Android mobile platform. It is the successor to the highly popular custom ROM CyanogenMod, from which it was forked in December 2016 when Cyanogen Inc. announced it was discontinuing development and shut down the infrastructure behind the project. Since Cyanogen Inc. retained the rights to the Cyanogen name, the project rebranded its fork as LineageOS.

LineageOS was officially launched on December 24, 2016, with the source code available on GitHub. Since that time, LineageOS has been described as highly popular and forcibly developed; within 4 months from the initial announcement, LineageOS development builds covered more than 160 models of phone, and over a million users, having doubled its user base in the month February–March 2017.