I also keep a set of AA batteries. Floppy and CD drives. If you bring the CD and leave the floppy, you'll need to transfer a document via floppy. This is a form of Murphy's law. Power adapter for a car's cigarette lighter.
Many new airplanes have cigarette-lighter power adapters under the seats.
Cat 5 cable I've found myself in hotels that have a strange lamp with an extra phone jack and an rj-45 jack. For a fee, I can plug my cat 5 cable into the rj-45 jack and connect to the hotel's network. Always having an extra bit of cat 5 cable has bought me Internet connectivity four times faster than if I'd left that cable at home. Dongle or extra phone cord. Two laptops ago my modem used a dongle to connect to the wall jack.