For work, it's my bespoke spreadsheet that automatically calculates when and how much I need to have on hand for any particular item. Our system is technically able to do this, but no one has ever turned it on, so I created one myself in order to get my inventory under control.
At home, it's probably the blender file of my current project for X-Plane that represents roughly a year and a half of my life so far. I have backups in multiple places of course.