This isn't exactly emulation, but I think this community would like it anyway.

I ported the vintage Mac game Continuum (asteroids in a maze with gravity, basically) to the browser. This is a full port--written in Javascript--but it sticks extremely closely to the original game logic so the port looks, feels, and sounds like the original. This was possible because the original authors released the source code about ten years ago.

Here it is: https://continuumjs.com

All the code for the port is on https://github.com/sam-mfb/continuum.

Enjoy! Use your shields often!