Gameduino is a game adapter for Arduino - or anything else with an SPI interface - built as a single shield that stacks up on top of the Arduino and has plugs for a VGA monitor and stereo speakers.
The sound and graphics are definitely old-school, but thanks to the latest FPGA technology, the sprite capabilities are a step above those in machines from the past.
You'll need controllers (most likely) for gaming, we suggest using Nunchuck with a Nunchucky Breakout
Video output is 400x300 pixels in 512 colors:
The adapter is controlled via SPI read/write operations, and looks to the CPU like a 32Kbyte RAM. (Unlike many 8-bit machines, there are no restrictions on when you can access this RAM). There is a handy reference poster showing how the whole system works, and a set of sample programs and library.