This video shows the R4i 3dS and CycloDS from http://www.r4i.ca playing some of the best NES games on the Nintendo DS lite. It works on the DSi, DSi XL and Nintendo 3DS as well. You use the r4i which lets you load homebrew and freeware games and emulators to load nesDS to the card. You pick the games you want to play and put them on the card as well, and your DS or 3DS turns in to a fully functioning portable NES system, with amazing graphics and sound.

What you need to be able to do this:

1. A nintendo DS, Dsi or 3DS (of course :) )
2. An R4i 3DS or CycloDS card (or an R4 3DS card - these work too)
3. The freeware nesDS emulator which you can download by doing a quick google search.
4. Some games you want to play that can be copied over to the r4 card
5. LOADS of time to kick back and play some of your favorite classic games :)