Exploring Commercial Products for the NES Expansion Port
tedium.co
tedium.co
There have also been several NES emulators that support online play.
Some things you can do with this include: - RetroAchievements, as you mentioned - Automatic timers/splitters for speedrunning - Co-op mode for Super Metroid: https://m.youtube.com/watch?v=ilG70fc-IJA - Multiworld co-op randomizers for games like Super Metroid or A Link to the Past -- two or more people play linked randomizer seeds over the Internet, and the items are distributed across games so you can find items for other players in your game: https://archipelago.gg/