The Bose QC15 headphones do an amazing job of cutting down airplane cabin noise. Everytime someone tried them on, their eyes went wide in amazement. They even did a better job of cutting down noise than plain old earplugs.
The best part about the QC15s is this:
The end with the switch plugs into the headphones. The "hi" setting is for your everyday MP3 player, cellphone, etc. The "lo" setting is for the airplane sound system. It keeps your ears from being blasted when the pilot or cabin crew make an announcement while you are watching a movie.