Hardware mods are cool, and for Nintendo fans few are cooler than an NES cartridge converted into a full-blown NES emulator system, including controls and a screen.
The result of efforts by Ben Heck forums member darkeru, the mod features a "cannibalizied" OneStation emulator, which includes 99 built-in games,mounted inside a gutted NES cartridge. The device runs on 3 N-size batteries mounted in the card-edge connector compartment, with a micro toggle switch for power. Select and start buttons are mounted on the sides, while a standard Gameboy-style directional pad and two buttons are mounted up top. In a forum post, darkeru described the process as "actually pretty simple," consisting mainly of moving around the various components to make everything fit into the restricted space.
Hardware modders make up a small, dedicated and perhaps a bit offbeat gamer subculture, somewhat similar to automotive enthusiasts with a taste for tuner cars. Some modding, particularly with PCs, is done to improve performance and can be as simple as drilling holes in cases for improved airflow and cooling, but more commonly the results are entirely cosmetic. The modding community is inherently competitive, but also very open; modders are usually quick to share their secrets (and show off their talents) with photographs, explanations and often even detailed tutorials of their work.
More pictures of darkeru's "Cartridge Boy" along with discussion about his work can be found at benheck.com.