I once wrote a MIPS assembler+VM in Racket. It went OK but I never used it for anything: https://github.com/gcr/mips-simulator
Was a great learning experience though.
Also, let us not forget the Halt and Spontaneously Combust (HSC) instruction: ftp://reports.stanford.edu/pub/cstr/reports/csl/tr/86/289/CSL-TR-86-289.pdf .
I once wrote a MIPS assembler+VM in Racket. It went OK but I never used it for anything: https://github.com/gcr/mips-simulator Was a great learning experience though.