There might be more interesting ways to answer this question, but the one item I never, ever leave home without is my Moleskin Reporter notebook (1 notebook, $10.95; pack of 6 or more, $9.95; moleskineus.com. The basic black hard-covered Moleskine is considered a cult classic thanks to the long lineage of wandering scribes who used them, from Ernest Hemingway to Bruce Chatwin. Because of their following, cynics might say the Moleskine is an overpriced marketing tool, but the notebook truly is the best tool for scribbling in the field.(Unless you work in the jungle, in which case you'll want to check out Rite in the Rain (riteintherain.com) notebooks.
I love the 5.5"x3.5", 192-page lined Moleskine for three reasons: I can write on every side of the acid free paper, it jams into my pants pocket when I need to be hands free, and it comes with a handy little pocket in the back that's perfect for collecting business cards, receipts, restaurant menus, or any other memory-jogging memento you need from your trip. Couple the Moleskine Reporter notebook with a Uni-Ball Vision Elite bold-tipped black ink pen that's guaranteed not to leak in flight and I am a happy traveler. But, there is one caveat: There is no free space on the spine or cover for a label.