I've created macOS versions of a wide selection of classic computer and console emulators, covering all the major systems from the eighties and nineties. These are available free of charge from this website. I'd love to list them on the Mac App Store, but unfortunately this is not possible.
Emulator Enhancer is a shareware module which adds a range of additional features and options to these emulators. Please note that use of this module is optional, and once installed you can try for as long as you like before purchase.