The one cartridge I want to know more about (that I never owned myself, but was a huge influence on me) is ISEPIC for the C64 (I always pronounced it "I-Sep-Ick" which obviously is wrong). From what I can put together myself after the fact, it dumped the loaded game from memory to disk and included a fastloader to read that dump back into RAM. I had hundreds of games on floppies that my cousins mailed to me monthly. In particular, I remember Beach Head II - the dumped game ran fine, but when I tried to duplicate the floppy it was on, I ended up with a long message about how pirating games was wrong, and then my computer crashed.

I never had the cartridge myself, but the cartridge wasn't necessary to play the dumped games.