I do a lot of work in cafes with Internet connections, and on occasion it can get quite noisy. My $20 Sony behind-the-head earphones work okay, but they're not really designed to block out a lot of that extraneous sound.
So clearly, what I really need are the new Bose QuietComfort 3 headphones. Like the previous incarnations of the QuietComfort series, these fellows have acoustic noise cancelling: they actively attempt to compensate for sounds in your environment. I tried on a pair of Bose sound-cancelling headphones several years back, and was amazed at how quickly my friend's voice turned into a mere murmur. Kind of made me wish I had them all the time.
The QuietComfort 3 features a smaller design than its predecessor, the QC2, and uses rechargeable batteries as opposed to the QC2's AAA battery. Bose also claims the QC3 uses better noise cancellation that the QC2, leading to a more isolated music experience for the listener. For another $40, you can even add a kit with a microphone to turn it into a cellphone headset.
Personally, I like the fact that you can wear the headphones with no audio device at all, if you simply need some quiet. It's a great way to deal with annoying co-workers or roommates.