DeSmuME (formally known as YopYop DS) is an open source Nintendo DS emulator for Linux, Mac OS and Windows. Its name is derived from emu which is short for emulator, DS and ME. It's possibly supposed to mean "DS emulator for me".
DeSmuME supports save states, the ability to increase the size of the screen and it supports filters to improve image quality. DeSmuME also supports microphone use on Windows and Linux ports, as well as direct video and audio recording. The emulator also features a built-in movie recorder.
The official homepage for DeSmuME is here.