DSemu - Nintendo DS Emulator
DSEmu is an open source Nintendo DS Emulator written by Imran Nazar and released under the BSD License. Unfortunately Imran hasn't been able to continue development of it and on the DSEmu forums he welcomed other people adding to it.
Although DSEmu is not very complete it does have at least two things going for it. The first is the source is available. And the second is it has a debugger and stepper built in. So you can step through the ARM7 and ARM9 disassembly which is quite useful.
I got the latest source and made a few changes to get it working with some demo programs. It has quite a few limitations still but being able to step through the code, examine the ARM registers, etc has prove quite educational to me. I've made that release, with source, available here for anyone who wants to use it.
If you're looking for a more featured emulator for running demo's and developing against you should probably use Dualis or iDeaS. They are much more full featured and polished.