Port of DeSmuME DS Emulator for the 3DS.
It's currently a fully interpreted DS emulator, which means the performance is not so great.
Put your ROMs in "root:/DeSmuME/roms"
Assign your ROMs to the buttons shown:
Default (no buttons held) - default.nds
L - L.nds
R - R.nds
ZL (New3DS only) - ZL.nds
ZR (New3DS only) - ZR.nds
A - A.nds
B - B.nds
X - X.nds
Y - Y.nds
Start - Start.nds
Select - Select.nds
D-Pad Up - d_up.nds
D-Pad Down - d_down.nds
D-Pad Left - d_left.nds
D-Pad Right - d_right.nds
Circle Pad Up - cp_up.nds
Circle Pad Down - cp_down.nds
Circle Pad Left - cp_left.nds
Circle Pad Right - cp_right.nds
C-Stick (New3DS only) Up - cs_up.nds
C-Stick (New3DS only) Down - cs_down.nds
C-Stick (New3DS only) Left - cs_left.nds
C-Stick (New3DS only) Right - cs_right.nds
Create an IMG file in WinImage, and save it to "root:/DeSmuMe/SD.IMG".