Up until now, Bose's Quiet Comfort 2 headphones have reigned in the land of noise cancellation. They're still king, but they're locked in an ivory tower, at the rarely discounted price of $300. Sony's MDR-NC50s do the job, and can be found for much much less than its $200 list price. A Froogle Search turns up pricing, from recognizable stores, as low as $125. (If you always hit the lowest bidder, no matter who it might be, you can get it cheaper still.)
So, for less than half the price of the Bose, Sony offers an elegant pair of headphones, with glossy piano-black "cans" to cover your ears, and a detachable wire to connect to your source. You only need a single AAA battery to power it up, and it will last up to 30 hours. Since noise-cancellation uses microphones to listen to the environment, there's also a "Monitor" button that sends through everything outside, so you don't have to take off your headphones to tell the flight attendant what kind of beverage you want.
Noise cancellation doesn't seal off everything, but it does create a calmer environment for you to hear your own music. The recreation of music by the MDR-NC50s is clear and full pristine in fact. It could be louder, however.
For contrast, I tested another pair of $200 noise-canceling headphones, the Solitude headset from Outside the Box, Inc. It wasn't as clear a sound, but it did provide more juice, owing perhaps to the second AAA battery it required. Besides a little extra volume, the Solitude pair offered something else that the Sony could use: a collapsable frame. Like I said, the Sony pair is beautiful, but it takes up a lot of room. The Solitudes fold up into a bag that's about half the size of the Sony.
Still, the Sony has the Solitude beat in sound quality, and for smart web shoppers in price.