Mac: OpenEmu is an all-in-one video game emulator where you can emulate your games, create collections, set up controllers, and plenty more.
OpenEmu does pretty much everything you'd expect. You can set up controllers with different layout schemes for each console you're emulating, organize your games by console or into different libraries, and of course, play games. OpenEmu's powered by open source emulators, which means it can emulate Nintendo, Super Nintendo, Genesis, Turbografx-16, and plenty more. It's also incredibly easy to use. OpenEmu scans your hard drive for games, supports a ton of popular controllers out of the box, and automatically pulls the box art for you. Basically, it's the last emulator you'll need, and it works great.
OpenEmu (Free)