I remember saving a lot of allowance so i could buy the Multiface One for the ZX Spectrum. Instant hacking and saving possibilities.

https://en.wikipedia.org/wiki/Multiface

I never had one, and often lamented it!

Being able to pause a running game and peek/poke at the RAM would have been very useful for hacking games, though of course I'd still need to crack the loader to share the POKEs with other people.