It was the "IndependenceCompiler" I used long ago to add the memory card exploit. You just set it up to match one of your original PS1 games, and it made a savegame file which you copy over with action replay.
Then with the memory card in, booting that original ps1 game would load the exploit on the memory card which in my case was either ElfLoader or HDLoader. (Depending on button held down).
Also allowed coping isos over to the the hd via ftp too over the nextwork which was handy