FeOS

FeOS is a free, open-source operating system for the Nintendo DS. Its aim is to provide a usable abstraction layer that builds on top of said platform. Because the Nintendo DS is a very resource-constrained platform, a port of a multi-platform OS is utterly pointless, even if said OS is designed for embedded platforms. Therefore, the OS must be specifically written for the DS.

Features:

Support for FXE2, a relocatable & lightweight executable/library format.
Two modes of operation: Console and Direct mode, the latter allowing for direct access to the Nintendo DS hardware.
ARM7 code loading support.
Cooperative multitasking through the MultiFeOS library.
Wi-Fi support through FeOSWifi, a port of the well-known dswifi library.
3D support through FeOS3D, a port of libnds' videoGL module.