melonDS is an open source Nintendo DS(NDS) emulator. melonDS is part of a second wave of DS emulators. There are a few other emulator projects that are part of it, which you might have already heard of. Either way, check them out!

melonDS Features:
* almost-complete 2D graphics
* 3D graphics (software renderer) with texturing, alphablending, lighting
* input: keyboard and joystick, touchscreen
* backup memory
* booting from the BIOS
* resizable video output window, preserving aspect ratio of individual screens

melonDS Changelog:

fix possible crashes when exiting
wifi: add basic access point melonAP (very beta)
fix SMULWx/SMLAWx opcodes, fixes Sims 2 music
add support for loading BIOS/firmware files and config file from AppData or ~/.config/melonds or from the executable directory (hcorion)
add 32-bit IPCSYNC reads/writes (Dirbaio)
add savestates
3D: (hopefully) fix bug in shadow/AA interaction (visible in the MKDS character select preview, bottom border of the platform thing)