Hacking the PET
A chip design company I worked at used PETs, even when I was there about 5 yrs ago, to run a couple of bits of custom test equipment that got used about once a year. It was just not worth rebuilding it to update it.
The engineers claimed to run some of the code on the floppy drive controlers and to use the floppy drive buffers as memory to get the performance they needed.
Now that's programming!