The last two cabinets I obtained, the previous owner stored a small assortment of spare parts (and a few junked parts as well, apparently he used it as a trash can in a pinch). Off the top of my head, one cabinet had a few cherries, screws, an extra joystick handle (a Wico from the looks of it) and a couple of manuals, the other cabinet sported some screws, extra lengths of wire (useless aluminums, bleh), some nuts and bolts, a hex wrench and a manual. I ended up discarded most of what I found since most of it had water damage.
I don't really store anything inside what few cabinets I have unless what I store in there is specific for that particular cabinet. Manuals, unique replacement parts and spare IC's are things I would likely store inside the cabinet. Once I complete my WIPs, I plan on attaching a small pouch inside all of them with a burned CD of all the notes and related files I might have for that particular cabinet. That way I can refer back to them if I ever have to do something with that cabinet.