Thinking about old video game consoles and arcade machines (very old, like those in the 70's/80's) it came to our minds what can be done today using very low-cost microprocessors. Generally,

NXP PH6030L | FET. PH6030L
N-channel TrenchMOS logic level FET
Rev. 01 — 29 July 2008

Product data sheet

1. Product p.

I have reproduced a B&W version of the classical game TETRIS running for either NTSC or PAL composite TV output.
No needs of special game shields or further supporting IC, but only a bare Arduino Uno and few other components (buttons and resistors).
It is a "conversion" plus some improvement of my previous color version for VGA monitors, shown here:

https://www.youtube.com/watch?v=qI0yp16fogI

Finally I put the Arduino in a pre-existing box with buttons I got from some scrap, but you can built your own as you prefer.

Code and instructions will be available here:
http://www.instructables.com/id/Arduino-Tetris-TVout/

We successfully produced 640x480 video and blinking colors from a non-overclocked 8-bit 16 MHz Arduino. See the theory, schematic, and full sample code.