I have a round griddle that was warped enough it was almost un-usable on my glass top stove. Here's what I did.
Using a ruler and an angle grinder, I knocked down the high spots. When I put it on the stove again it was significantly better but still a bit wobbly so I took a cheap 2-sided sharpening stone and continued taking it down with that. It took a few hours to get as flat as I wanted but, I felt like it was worth it.
If we're talking about things I use regularly, probably my desktop computer. I bought it at an auction. This was in '04 and have been upgrading it one piece at a time ever since. I'm currently gathering parts for another "gut it and start over" level rebuild and I think the only original parts at this point are the case and the floppy drive.
If it's just things I own and don't use, I do technically own a '76 Chevy pickup that is almost infinitely repairable but that hasn't had an engine since the early 2000s