I love in-ear headphones ("canalphones"). These are like earbuds, except they go into the ear canal, the way ear plugs do, providing excellent isolation. They are perfect for air travel, all but curing the fatigue one feels from the constant thrumming of the engines.
Happily, Dan Frakes wrote an article for MacWorld that does a great job describing in-ear phones, so I don't have to! See: In-Ear-Canal Headphone Primer.
What I love: Great sound, incredible isolation (much better than "noice-canceling" types), very compact.
What I don't like: Uncomfortable until you get used to them, so isolating that they cut you off from the world, inconvenient to insert and remove if you need to talk to someone. Shure has a cool gadget now that has a microphone and a button — push the button and it mixes in the ambient sound, so you can hear without removing the 'phones.
I heard a set of Shure E5s at a show and they are fantastic. They are also (gulp) $500. But you can get good ones for $50-100, too.