A lot of monitors, the older ones definitely because they lacked any kind of degaussing system.
http://en.wikipedia.org/wiki/Degauss
The wikipedia provides! All hail the wikipedia! Hail!
Monitors without a degauss function can be degaussed by placing a degauss-enabled monitor face-to-face with the target monitor. Activating the degauss function on the enabled monitor can help degauss the other. Alternatively, one can affix two oppositely poled magnets to either side of a drill bit, attach it to a drill, and then hold the spinning drill to the target monitor.
Degaussing can also help reduce the distortion due to static electricity that often results from long term usage of the monitor, especially in older monitors