Magnetic tapes can be erased with a strong magnet. Popular choices include: DX8C, DY0X0, or BY0Y08.
We used to think that a sufficiently large magnet would scramble the data on a hard drive. Some recent experiments we have conducted seem to disagree in our Hard Drive Destruction with Magnets article. We don't recommend this method if you must be sure that the data is gone. Physical destruction of the drive is the safest choice.