Yeah I have a flashcard. I use it to play my hacks of Super Mario World, Super Mario Brothers 3, and Pokmon Blue, Yellow, Gold, and Crystal.
Oh
Although theres plenty of other things you can do:
Play emulators-You can emulate gameboy color, nes, snes, and a few other old systems.
Play music - Add songs and turn your DS into an MP3 player!
Watch Videos - Put videos on your DS such as movies (probably WAY too big to fit a FULL movie =O)
Play DS games - Launch DS roms straight form the card! (WARNING: DOWNLOADING ROMS IS
ILLEGAL UNLESS YOU OWN THE GAME!)
Ported Games - There are a few old games that are ported to the DS such as: WOLFENSTEIN 3D! (Single handedly started the FPS gaming!) DOOM 3D (Same engine as Wolfenstein) Quake! (New engine based off of Wolfensteins original code) [People are modding Quake DS to be like halo! Its called iHalo DS! Its a great little game.
Custom Games- People make cool homebrew games ALL the time such as little racing games, simple shooters, pacman etc.
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