Nintendo DS emulators have been around for awhile on PCs, but a proper touchscreen version for a mobile platform like Android OS? That's new, and in this case, not a gimmick like the "shell" iPhone app DS Double Sys. This time we're talking about an actual handheld-based DS emulator.
Okay, it's not really ready for primetime, and in fact designer Tiger Lab calls it a "very early prototype version (beta) for demo only," but hey, you want a DS emulator on your Android phone, Tiger Lab's put it up for download (though only "for limited days").
Tiger Lab really just wanted to demonstrate it could be done and says the emulator is "very very slow so you shall not expect to play NDS game [sic] with this emulator." Translation: You'll need a pretty powerful Android phone to get it working, and even then, you'll want to "just try some small [non-3D] games."
You'll also need Nintendo DS ROMs, software copies of the original Read Only Memory chips used in Nintendo's video game cartridges. They're also generally illegal, and whether you own a copy of the original game or not, downloading them's a legal no-no.
Tiger Lab says it plans to pour "huge efforts" into getting the DS emulator up to grade by adding a dynamic recompiler, simulating a graphics processor (GPU), allowing OpenGL hardware rendering, and adding a floating point unit (FPU).
Incidentally, the game on the right in the image up top is obviously Mario Kart DS, but the game on the left? I had to dig, but apparently it's a shoot-em-up called Geo Wars released back in 2007, though I'm pretty sure you need an R4 or M3 card to play it.