A while back, when I was doing Nintendo DS homebrew, I got myself mentioned in a book! Remember those? This one was Chinese so I have no idea what it said, but I was in at all the same!
I basically wrote a tool to test most of the functions of the DS so you could see if a button was failing, or if maybe the touch screen was misaligned, that sort of thing. I gather that it was quite a popular app.
Here is the source code for it… DiagnoSe_palib_source
and here is the .nds file, along with manual etc. Diagnoseds