Something that has become a regular trend in the modding world is to take an older video game system, and make it portable. The most popular console to do this with is the Nintendo 64 which has been molded into many different styles and variations as a hand held device. This particular version is my favorite as the person who created this system made pretty much everything down to the plastic case.
These handheld Nintendo 64 systems are typically made with a 3.5" LCD screen and (if the case can hold it) a battery that can generally last around 3 hours. Now granted, these are made by people in their homes, so there's no real rules on how to make one of these.
Another great feature with this particular make is the ability to play with up to 4 people just like the original N64. Of course the screen seems too small to fit a round of 4 player golden eye, but it's the thought that counts. I personally would want to integrate some sort of output for TV play, or perhaps a dock that you can slide this nifty system into. So, does this thing work? You bet!
The portable Nintendo 64 is definitely one of my favorite mods and a fantastic project. I'm going to set aside a small fund to one day make one of these bad boys myself. If you want your own portable N64 but can't/don't want to make it yourself you're in luck since a lot of the people who make these also sell them. However, you'd have to bust out your check book since most of the time the asking price for one of these nifty portables is around $1,000.