Yes there are many homebrew games in the homebrew browser
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You can learn to hack various video games for pretty much any
console, at GameHacking.org (see Related Links below).
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Previous answer:
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Using a homebrew or hacking device to alter the games rewritable
software
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no you can not
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homebrew
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It is not illegal in the sense that you use it for playing mp3,
or playing homebrew video games that you have created yourself. It
IS, however, very illegal to download copyright games and play them
using the m3ds.