Emulation / Playing of Nintendo DS ROMs
There are two basic ways of emulating the Nintendo DS.

Software Emulation (external link) - This is the free way to play the DS. You need a good (fast) PC, and most games can not be emulated completely at this stage.

Hardware Emulation - This is the easy way to play roms on the DS. You need a Nintendo DS and a second device listed on this page. Once you have these, playing roms is as easy as copying and pasting roms to your DS.