OpenEmu is a free multi-platform retro video game emulator for OS X. It has emulators for Atari, Nintendo, Sega, PC, and Sony consoles. Setup is brainless Once you install it, you can drag and drop your ROM files (making sure, of course, you legally own them) into the app and start playing them. You can play the games from your keyboard, but I recommend getting a USB controller. The one I use is this Buffalo Classic USB Gamepad:
To configure it, go to Preferences in OpenEmu, click Controllers, select the controller you want to emulate, then follow the prompts for which buttons to push.
You'll be playing your favorite old games in no time. Check out the large library of great homebrew games that you can download directly from the app, too.