NESEM
I was motivated to write an emulator back in 2008 and have been occasionally hacking away at it every since, through various languages and years.
This originally started as a project in VHDL when I was taking a hardware class, and I got as far as dumping the sprites onto a VGA monitor but then I had to return the dev board I was using. Then I tried it in Java for a bit since I didn't know graphics in C. After a while I decided to learn C++ and at the same time learned OpenGL and SDL, so this has been using those ever since.
At this point, it can play a basic game of pong, though some of the sprites still don't render properly. I haven't done any sound work, and the graphics need some work still. Who knows if I'll ever get around to implementing those. It also doesn't limit FPS at the moment, so it's a challenge to play :P
My challenge to myself was to do it withouth looking at code from other emulators, so this is done purely from reading the various PDFs and wiki pages floating around.