Nintendo DS Hardware Emulation
This isn't really emulation per se, because effectively you are playing the game on original hardware.

Slot-1 Devices:
M3 Simply / R4DS / CycloDS
These devices are pretty much identical. Everything between them are compatible except for the firmware.
Features:
- Uses MicroSD cards (upto 4GB per card)
- Can play almost all DS roms directly
- Can play movies & music (via MoonShell)
- Can play homebrew without DLDI patching (via R4 Firmware 1.10)

Links
M3/R4 Skins
M3/R4 Game Compatibility List

The R4 is generally a little cheaper than the M3, and firmware upgrades are available first on it.
DS-Downloads recommends the R4DS

DS-Xtreme
Features:
- Integrated memory chip - either 512MB (4Gb) or 2GB (16Gb)
- Plugs directly into a PC USB port
- Comes with some pretty mediocre applications
- Complete, one piece solution.

G6 Real
Features:
- Standard 'Direct boot, roms, movies, music' compatibility.
- Upgradeable Firmware
- Comes with a USB adapter and works like a Flash Drive (Drag n' Drop)
- Built in PDA software

Slot-2 Devices
These devices plug into the GBA slot, and natively play GBA Roms. They require an additional device called a PassMe to play DS Roms. These are generally a lot more expensive than Slot-1 devices. The ONLY advantage is that they have 100% GBA compatibility.

G6 Flash Lite + PassCard 3
Features:
- Supports compressed (.zip) files
- Comes with USB connector
- Non-upgradable firmware (beware!)

SuperCard & SuperKey
Features:
- Many formats compatible (GBA, NES, GB, PCE, SMS/GG)
- Plays GBA movies, books, music, images
- Good, all round arcade machine

DS-Downloads Warning: Stay away from the EFA (aka. Extreme Flash Advance/ Longcard). This is a very poor quality product. Our sample of this broke multiple times and finally stopped working less than 6 months after recieving it.