Loopy (A NDS homebrew developer) has started on creating a custom firmware for the NDS system. At the current time this firmware is very limited and would/should only be used for developing purposes and not for oneself to try out.
So, why does the NDS have custom firmware? With the creation of flashcarts, developers have been able to provide more content on the Nintendo handhelds and have improved the system. The NDS has no firmware protection and can thus be flashed with just a few tools handy (screw driver, passkey device and a flashcart). To learn how to flash your NDS, read my guide: [here]
source: nintendev.comOriginally Posted by Loopy