I'll make a recapitulation of existing DS emulators, since the question is recurrent and it won't do any harm to have it in one place.
iDeaS - a nice emulator, the latest version being of June 6th.
Code:
http://spazioinwind.libero.it/linoma/ideas_pages/downloads.html
DeSmuME, latest version - July 10th:
Code:
http://www.desmume.org/
Here is the ROM compatibility list: http://desmume.org/?page_id=15
No$gba, has a few WiFi possibilities...
Code:
http://nocash.emubase.de/gba.htm
DSEmu, an Open source emulator:
Code:
http://www.double.co.nz/nintendo_ds/dsemu.html
NeonDS. Does a great job.
Code:
http://www.neonds.com/en
The ROM compatibility list: http://www.neonds.com/wiki/index.php?title=Main_Page
And finally, Dualis
Code:
http://dualis.1emu.net/index.html
If you know another good emulator, please share.
Cheers