In the last several years I've boarded over 300 flights and have consequently learned quite a bit about packing my carry-on luggage strategically. This is a list of 10 things I always include in my carry-on bags.

1. Passport, driver's license, international calling card and credit cards: This seems pretty obvious, but before heading to the airport it's always a good idea to check to make sure you've included these and that they're not at the bottom of your bag. In fact keeping them all together in a secure spot in your bag can be a good idea, because then you'll always know where to find them. Plus, you'll need to show at least one of them when checking in and again when you reach the security gates.
2.  Medications: This applies to everything from Tylenol to prescription meds. Definitely include these in your carry-on bags. You'll be thankful for some Tylenol after the baby next to you has been crying for half the flight, or when you arrive in a new destination and your allergies mysteriously start acting up because of the air or the grass or any number of things. Also, if your luggage accidentally gets shipped elsewhere you won't spend your time in a foreign country searching for a way to get your prescription migraine/heart/diabetes medicine sent to you.
3. A little cash: A little spare cash is always a good thing to carry. Most airports have cash machines, but sometimes they're not working or they're closed or they simply don't exist. Having some cash (in a variety of small bills) for your arrival can prove helpful, especially if you need to catch a cab.
4. A book and/or magazine: Reading a book or a magazine can be a great way to pass some time on a flight or during a delay. I always stuff at least one book into my carry-on bag and if I'm bringing a travel guidebook I also pack it in the carry-on, along with my destination info, so I know where to head once I get off the plane.
5. Ipod: I know this isn't a necessity, but I like to travel with one. My mom used to think Ipods were silly until she got one for the holidays and I downloaded a book, a few songs and some Spanish language guides onto it. She was thrilled to discover that she could knit on the plane and simultaneously listen to a mystery novel without toting around a bulky CD carrier. My ipod music sometimes helps me fall asleep on a flight. Sometimes I like to put on the headphones without turning on the Ipod just to drown out the plane noise, and I've also noticed that when you put on headphones people leave you alone for a bit.
6. Hand sanitizer: Like the name says it sanitizes your hands. Not only is this a good thing to have on airplanes (which, let's face it, aren't exactly the cleanliest of places), but if you're going hiking or camping or venturing off the beaten path (like I often do) trust me you'll probably be glad you have a little hand sanitizer in your bag.
7. Emergen-C or Airborne: According to a recent study in the Journal of Environmental Health Research, people are 100 times more likely to catch a cold on an airplane than during a normal day on the ground. I take a couple Airborne tablets or Emergen-C packets in water before I get on the plane, and I like to take them during my trip as well because they help support the immune system.
8. Bottled water: Since we can't check water through security any more, I like to buy one before getting on the plane. Air travel dehydrates your body, and it turns out that water can also help counter a number of common travel ailments including headaches, cramps and fatigue. 
9. A plastic bag of essential toiletries: This is where I put my toothbrush, some toothpaste, sunscreen, facial cleanser, face cream and lip balm. I'm not big on traveling with a lot of makeup, but a few essentials (like sunscreen) can really come in handy. If you really want to travel with makeup, I'd suggest going with a little 3-in-1 travel compact that includes an eye shadow, lip color and blush.
10. My video camera: I always take this on the plane with me, because I want to make sure it arrives at my destination safely. Sometimes this means leaving some other items behind if everything can't fit, but as a video journalist my camera (and the tapes that go with it) are pretty important to me. So if you have something that you care about greatly or is valuable to you, take it on the plane with you.