The Commodore 64 had some popular cartridge expansions. The most popular were in my humble opinion KCS Power Cartridge and Final Cartridge III.
The extra features those cartridges gave the C64 were amazing.
I have several Final Cartridge IIIs in my collection but only one complete in box.
Information:
https://www.c64-wiki.com/wiki/Final_Cartridge_3
https://en.wikipedia.org/wiki/The_Final_Cartridge_III
https://www.breadbox64.com/c64-peripherals/final-cartridge-iii/
https://www.pagetable.com/?p=534