I’ve come across various websites that offer Nintendo DS emulators as free downloads. The following are sorted according to compatibility with commercial ROMS (source http://www.ndsemulator.com/):

DeSmuME (download)
DS iDeaS
Nintendo Ensata (download)
DualiS DS (download)
DSemu

For a complete list of other console emulators, visit this website containing list of video game console emulators from Wikipedia.