I love playing doom in SymbOS ;)
Yes, you read that right, a proper first-person shooter, running on the very same kind of Z80 chip that used to power my old MSX. If you had told ten-year-old me that this would one day be possible, I am not sure I would have believed you. It is, without a doubt, one of my favourite ways to show off just how far this little community has managed to push these old machines.