This DIY portable game console features a color screen, tilt sensors, and 4 games. Of course you can always program more games if you build one for yourself. Check out the link for pictures of the system stuffed inside a relatively small enclosure. This is a great electronics project that is very well documented.
By pulling together a handful of low-cost parts and building the system ourselves, we have demonstrated that a complete programmable portable console can be had for less than $70, even when purchasing the parts we sampled. Our implemenation runs on a single 9V battery and features tilt-based control, monophonic sound, and 3 playable games. We call it the Weeboy because it combines features of the Nintendo Gameboy Color and the Nintendo Wii.