I did some Mame hacking the other day. Basically, I added code that would let me automatically reload a state and record my inputs at the touch of a button on my fight stick. I wanted this functionality so that I could test various glitches in games and then replay and record the event.

While I'm not the biggest fan of Mame's source code (it's still got a lot of macros from it's C heritage), it is fairly simple to understand, and you can gain a lot of insight into how the hardware worked back in those days. I hope this project lives on for a very long time.