Commodore PETvet
If you are lucky enough to have been blessed with owning a Commodore PET computer then you know these machines can be cantankerous.
Thanks to the efforts of Mike Hill you can diagnose problems with your PET and halt the processor / view the memory with a terminal program.
You can also customize the memory with your own data.
If you own a PET this tool should definitely be in your arsenal.
You can see an overview of the PETvet that took place during the 2012 Vintage Computer Federation conference.