Not only regular magnets, like the kind you stick to your refrigerator, but things that have magnets in them! I wondered one time about the cause of my distorted screen, when I noticed that my roommate Jacob had moved his surround sound speaker, placing atop my monitor. On his return, I demonstrated the interaction those devices have.

Another time, my friend James was moving things around in his dorm room, and he got his 8-inch subwoofer a little bit too close to the computer. It crashed, and he lost quite a few sectors of data from the incident. I myself have set floppy disks in front of my stereo speakers, with varying results.