One cool and useful item that came with my NEC PC-6001 is this RAM/ROM expansion cartridge. It plugs into the side of the 6001 and doubles the memory to 32KB, and it also has two LIF (low insertion force) sockets for ROM expansion.

At this point, I’m not sure what can be done with the ROM expansion but I hope to find out more someday. As for the RAM, many tape games require it, so it seems like something of a must-have expansion for this system.