nds4droid by jeffq is a free, open source Nintendo DS emulator written for Android smartphones. It is based off of the excellent emulator DeSmuME. The source code can be found on sourceforge.

Changes:

* Added a new, much sleeker ROM browser (thanks Alexander P.)
* Added an autosave feature, on by default, which will save the game’s state periodically

http://jeffq.com/blog/nds4droid/