Although I have placed this in the Video Game Consoles category, MAME is not technically a home gaming machine, and is rather an emulator -- or a software package -- that allows you to play arcade titles past and present on a home computer.
Cartridge based systems are easier to emulate because the code can be extracted from the circuitry.An extensive emulator website, Emulator Zone provides all kinds of information about emulators, the history of emulation and downloads.
That said, if you choose to use a GameCube emulator (at your own risk and discretion), there may be several "hacks" and "mods" available on the Internet that can effectively work the same way as new GameCube codes.
While LoveToKnow doesn't condone the method of downloading copyrighted games to use with your Nintendo 64 emulator, many companies have released game code so that the games are compatible with emulators.
One of the ways that you can get whatever character sprite you'd like is to download an emulator, get your hands on the video game ROM you want, and then simply do a screen capture on your computer.
This emulator was first used to play homebrew games on the PC, but new releases increased the capability to play ROMs of popular titles like the New Super Mario Bros. and WarioWare Touched!
A cartridge based console system is easier to emulate because the data can be simply pulled from the cartridge and transferred into computer programming for the emulator to read.
You'll have many choices to play unique and one-of-a-kind games; many may not be that good, but it's possible you'll find a gem to play on your Nintendo DS emulator.
With some careful searching on the Internet, you'll find information about the MAME emulator, for instance, that can be used to run a wide range of video game ROMs.
Gens32 is another emulator based on the Gens source code and allows compatibility with all the major Sega releases, except for the Dreamcast and handheld systems.