Soft NDS Emulator is a free NDS Emulator, based on Smart NDS Emulator.

Soft NDS is much like the other free NDS emulators for Android. Sound is a bit glitchy and game play a little laggy but most games are fully playable.
Soft NDS did seem to run slightly faster than the other emulators, on my phone anyway.
There are pop-up ads for other free apps that are not too intrusive and do not interfere with game play.