Something I have here is MIJS: Mips In JavaScript which is a quick attempt to emulate a MIPS computer in Javascript. My initial target is the variant used in UWaterloo's CS241 class.
There's a host of other associated machinery that I'd like to write to make this cool, including a VT100 (or something) emulator that can run in a browser so you can interact with the system, and maybe some sort of API to access the HTML DOM from inside a MIPS program.
It's just a bit of an experiment and not really useful for anything. It pretty much works, except it has no I/O. You can look at "memory" though.