Some people like me are happy playing their NES ROMs on their Dreamcast or other emulation machine. However, there are the extra hardcore that insist on only playing NES games on a real Nintendo Entertainment System.
With an NES Flash Cart, you would have the opportunity to play all of the best NES games on your NES (original or a clone) without having to change cartridges and protect the original game carts.
I have found this project site in which this guy is planning on putting together a number of NES Flash Cart kits that you can put together yourself.
He is still trying to finish up the needed software and get other things rounded up, but he plans on selling them soon. Eventually you will be able to hook the cartridge up to your PC via USB and transfer your favorite ROMs to it. Then the cart will play just like a real game.
He has already had Super Mario Bros. 3 running off of one successfully, but he wants to make sure everything is near flawless before distributing them.
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