bitfixer's Pet Vet, and Nicolas Welte's Universal 6502 RAM/ROM Expansion. Both essentially do the same thing, they plug into the 6502 CPU socket, and selectively isolate the CPU from the rest of the system. They can be configured so that any ROM access uses the copies of the system ROMs (or even different ROMs) on the expansion board, rather than the ones on the main board which may be faulty. It does the same from RAM access, it can use RAM on the expansion board to replace and/or expand the main board RAM.
UPDATE # 1
Commodore Pet 4032 restoration.
UPDATE # 2:
new V2.0 6502 ROM/RAM boards are available now.
V3.1 ROM/RAM boards are available now
These boards are available to buy from my Tindie Store and TheFutureWas8Bit.