Occasionally, SEGA provides free classic games during special events like "Make War Not Love" (MWNL). If you obtain even one free game (like The Revenge of Shinobi or Streets of Rage 2 ) through these promotions, the SEGA Mega Drive and Genesis Classics launcher is typically added to your library automatically.
If you're looking for the complete curated experience with features like save states and online multiplayer, the official collection is available across several platforms. How to install mods with Sega Megadrive Classics (Tutorial) SEGA Mega Drive and Genesis Classics Free Download
You can find a variety of free, community-made games and demos specifically designed for the SEGA Genesis on platforms like itch.io . Where to Purchase the Full Collection SEGA Mega Drive and Genesis Classics Free Download