Batocera

Batocera.linux is an open-source and completely free retro-gaming distribution that can be copied to a USB stick or an SD card with the aim of turning any computer or nano computer like Raspberry Pis and Odroids into a gaming console, for one game or permanently. 

Batocera.linux does not require any modification on your computer. Note that you must own the games you play in order to comply with the law.