The Nintendo DS line had dual 4:3 displays. There was no way to change the aspect ratio, even on Wii U Virtual Console and 3DS. With some Action Replay codes however, this is no longer the case.

Note: These mostly effect the polygonal aspects of games. The HUD and other 2D aspects remain largely unaltered

Mario Kart DS - 16:9 [NTSC-U]
12147E04 00003D30

Mario Party DS - 16:9 [NTSC-U]
52036CD8 00001555
02036CD8 00001C71
D2000000 00000000

Mario Party DS - 21:9 [NTSC-U]
52036CB8 E59F3018
02013AF0 00002555
02036CD8 00002555
D2000000 00000000
5217CB2C E59F31CC
0217CD00 00002555
D2000000 00000000
52182360 E59F3C8C
02182FF4 000012AA
02182FF8 00002555
D2000000 00000000

Super Mario 64 DS - 16:9 [PAL]
520209C4 E59D0094
020209C8 E3A09C15
0200D03C 00001C71
D2000000 00000000
520B21E8 E59F00E8
020B227C E3A02B07
D2000000 00000000
5211530C E59F205C
02115370 00001C71
D2000000 00000000

Mario and Sonic at the Olympic Games - 16:9 (NTSC-U)
520506D8 E8BD81F0
020506D4 EA0BDB09
02347300 E3A04003
02347304 E0000490
02347308 E1A00120
0234730C E5850000
02347310 EAF424F0
D2000000 00000000

To-Do-List:

Zelda: Phantom Hourglass
Zelda: Spirit Tracks
Diddy Kong Racing