How small can an arcade machine get? Really small, it turns out, and really cheap, too. If you use a Raspberry Pi, that is. Last year we wrote about the PiCade, a Kickstarter project that used a Raspberry Pi to power a desktop arcade cabinet with a 12-inch screen. That project is still coming, but it's actually more complicated than a homebrew arcade machine strictly needs to be, since it includes a custom PCB, screen, and laser-cut cabinet. Adafruit, which sells all kinds of Maker tech, has a great guide to setting up an arcade machine with a Raspberry Pi as cheaply and simply as possible.
Adafruit's guide walks Raspberry Pi newbies through the entire setup process, which entails installing Raspbian (Linux for the Pi) on an SD card and configuring it with mouse and keyboard. Then comes the fun part: downloading MAME, Super NES, PS1, and Atari 800 emulators from The Pi Store, all of which are free. The guide also explains how to run these emulators from the console, which offers better performance. Remember that perfect SNES emulation can require a 3GHz clock speed, so the Pi (with a 700MHz CPU) isn't going to have the fastest or best emulation around.
The guide then details how to set up controls, which means getting a joystick and buttons to work with the Raspberry Pi. Adafruit sells an 8-way digital joystick and arcade buttons, though some gamers may prefer the Sanwa buttons used on Japanese arcade cabinets. Those will work, too, and Adafruit walks you through how to get everything hooked up to the Raspberry Pi board. Adafruit also sells some wires that make it quick and easy to get all of the buttons connected.
The last piece of the puzzle is Adafruit's Retrogame utility that talks to the software and handles all the control configuration with no fuss. Of course, if you do any level of customization or build in extra buttons--six for Street Fighter, instead of the two button setup Adafruit shows for classic arcade games--you'll need to play around with the software.
The walkthrough won't produce the fanciest arcade machine around, but it's small and easy and a great starting point. If you're interested in turning a Raspberry Pi into a retro gaming machine, check out another build, which includes Sanwa parts, keyboard emulation, and a woodcut cabinet.