I just got myself reacquainted with the Nintendo DS homebrew scene after enjoying so much excellent gp2x and gp32 emulation software. I was amazed at the level of emulation possible on the Nintendo DS with help of any homebrew software supporting device. A lot of the emulators can be 'patched' to support various DS 'backup' devices by injecting the proper drivers with the dldi driver utility, so no matter what device you have you'll probably be able to use an emulator on the DSLite/DS

Emulators on the Nintendo DS

FrodoDS30507 (dldli compatible)
C64 - 50-80% speed c64 emulation, on some games even close to 100%
Keyboard emulation and joyport selection is there. Not all D64 files are supported, Frodo is a little picky when it comes to that.
Troy Davis(GPF)
http://gpf.dcemu.co.uk

ColecoDS (dldi compatible)
Alekmaul
http://www.portabledev.com
Colocovision - 100% speed, close to or even 100% sound, those games really come to life on the DS screen.
The touch screen is used to input the numeric keypad.

Penkods-dldi-v0.2a (dldli compatible)
http://deadketchup.kyuran.be/
MSX 1/2 - getting to 100% speed, sound is not completely there yet
disk and romfiles are supported.

SpeccyDS 0.2a (dldi compatible)
http://speccyds.wordpress.com/
Spectrum - 100% speed, 100% sound, wonderful graphics on the DS screen. The whole spectrum keyboard is graphically represented on the bottom screen. All imaginable joystick interfaces are supported. SNA, Z80 files supported.

Pocketnes
Various incarnations on both gba and DS, later versions also function with various backup devices and quite a few devices feature the emulator in the bios.
Nintendo/Famicom emulator 100% speed, 100% sound, screen can be scaled. Wonderful compatible emulator

SnemulDSv05b (dldi compatible)
Snes emulator running at 100% speed and 100% sound in quite a few games. Now this emulator is simple amazing. I was totally flabbergasted by the compatibility rate and the fluidity of the gamegraphics, clarity of the sound. There are graphical glitches and a ton of settings to tweak, but this emulator sure is a big accomplishment on the DS.
Includes CPU core from SnezziDS (by bubble2k) and sound engine from PocketSPC (by gladius)
http://www.snemul.com/ds/

crocods-dldi-v1.0 (dldi compatible)
Amstrad CPC 6128 emulator - running at close to 100% speed and ditto sound. Works with disk images, includes a virtual keyboard very much like the MSX emulator. Quite a good compatibility rate.

These were the ones I tried out, but there's tons of nice new things on the horizon. A good place to start is http://www.portabledev.com/pages/ds/nds-emulation.php

MarcaDS : Emulateur de bornes d'arcades pour Nintendo DS
MarcaDS repose sur le même principe que Mame mais n'utilise pas son code source
A new multiple arcade emulator for the DS with the same design principle but not based on the mame source code.
Version 3.0 du 02/12/2006 emulates no less than 108 arcade games, the minority features sound

Please post here your experiences!