I was a (small) part of the Spectrum hackers clique in Warsaw. Most of us had "Masterface" which was a hardware extension that had a switch to replace the ZX RO

I was a (small) part of the Spectrum hackers clique in Warsaw. Most of us had "Masterface" which was a hardware extension that had a switch to replace the ZX ROM with a custom-baked one and used NMI to do things like snapshotting the state of the machine to tape or loading a custom debugger. The first one was used for pirating games (but pirating by snapshotting was considered cheap) but the debugger was the real deal. It fit in the screen memory (so as to mess as little as possible with the game memory) and let us discover "pokes" for infinite lives. I think it was called Fox Mon or Foxmon? Cool times.