Availability: 100% Free, Homepage

This is a mass system emulator with support for pretty much everything: PlayStation 1, N64, Nintendo DS, SNES, NES, Game Boy, Game Boy Advance, NeoGeo Pocket Color, Virtual Boy, Genesis/Mega Drive, Sega CD, 32X, Sega Master System, Game Gear, PC Engine/TurboGrafx16 (plus CD support), Wonderswan Color, Atari Lynx, arcade (MAME), and more. It offers lots of features: screen filters (shaders), save states, different screen overlays, cheats, netplay, and more! It's highly customizable! I highly recommend RetroArch.
*RetroArch is also available for iPads and Apple TV!
*Need help? I don't have a specific tutorial for RetroArch on iOS, but you can check out my RetroArch for Android tutorial. Both versions are nearly identical.

Availability: 100% Free

This is a mass system emulator with support for NES, SNES, N64, Game Boy/Color, Game Boy Advance, Nintendo DS, and Sega Genesis. Features include save states, cheats, gamepad support, and beautiful overlay skins for the onscreen buttons. You can sync your games, saves, and skins with Google Drive or Dropbox.

Availability: Free with ads

A simple, good PlayStation 1 emulator that offers save states, custom controller skins, and cover artwork for titles. The only annoying thing about it is that you're presented with fullscreen ads as you use it. If that annoys you, can you use RetroArch for PlayStation 1 emulation.

Availability: 100% Free

The iOS port of the #1 open source Sony PSP emulator! It's a superb emulator with many features. PSP is rather CPU-heavy to emulate, so don't expect games to run perfectly on your iPhone. Fortunately, PPSSPP offers lots of options to help you optimize performance to help games run smoother.

The most popular emulator on OSX. It's a multi-system emulator that supports around 30 systems! It has a nice and clean UI, too; as expected from the reputation of Mac apps. Make sure to check out its wiki for help and to explore all the features available. Don't forget that several systems such as PlayStation 1 and Sega CD require the BIOS files to work, which you can download below in Extras.

If you're not happy with OpenEmu or prefer standalone emulators, you can download all of them from the #1 website for video game emulation on OSX, MacScene.net! It has a pretty active community, too. • You might also want to check Bannister.org because that guy alone made the majority of the available video game emulators for Mac. In other words, he's like the father of video game emulation on OSX. • For some reason, MacScene.net doesn't have a PS1 emulator posted. The best PS1 emulator on Mac is DuckStation.

If you have a newer Chromebook with access to the Google Play Store then all the magic and wonder of Android's strong selection of video game emulators are at your fingertips. I've tested the popular emulators on Chromebooks and they all work great. RetroArch is a top emulator I recommend for supporting over 40 systems.

If you have an older Chromebook that doesn't have access to the Google Play Store, GPemu is for you! It's a multi-system emulator supporting Game Boy, Game Boy Advance, NES, and SNES. It works offline, has save states (right-click screen for access), and can load zipped ROMs. Emulation quality is pretty good with only a bit of scratchy audio. You need to manually enlarge the window to blow it up to a good size. Gamepads are not supported (they generally aren't on Chromebooks). You may want to check out browser-based emulators as well (they might fare batter than GPemu).

The easiest way to install video game emulators is to first install the Software Center (if you haven't already). There are plenty of videos on YouTube showing you how. Once installed, simply do a search for emulators to bring up the full list. Most of the emulators should work. Depending on your system and setup, some may not work. To install video game emulators manually without the Software Center requires a bit more than a beginner's understanding of Linux. YouTube is definitely your friend if you want to learn more about this. There are some really helpful videos out there, like this one: How to Create a Retro Gaming System with Ubuntu. Have you heard of the ultra-affordable Raspberry PI? You can easily turn it into a RetroPie: How to set up a RetroPie. So for $35 US you'd have the ultimate video game emulation system!