If you want to go legal Steam also has a few retro collections that I own that feature various emulated platforms including Snes, Nes and arcade.
Castlevania Collection
Contra Collection
SNK Arcade Classics
All decent enough emulation IMO from someone who played most of the original games on console or arcade.
Edit, checked my Steam list, there's more legit ways to play retro on Steam than I hadn't remembered including multiple emulated Nes and Snes games, it seems I have quite the legit collection of retro on Steam to add to my others:
Megaman Legacy Collection
Street Fighter Arcade Collection
Konami Arcade Classics
Capcom Beat em Up Collection
Add the Megadrive Collection on Steam and my Snes and Corefx Mini's, I've got plenty of legit ways to play retro games.
If they ever released Snes collections on Steam it'd be getting close to a really quite decent emulation platform in itself.
I have some too on Steam. The main collection that comes to mind however is the Mortal Kombat arcade collection. It was poor as I needed to fix it to even play it (windows games for live problem if i remember correctly) and the online multiplater was non existent. In contract I tried something years ago on the lines of project MUGEN or something like that. Online play and it worked great