Flights
Hipmunk: A slick, even cute, first stop, with easy comparison of options through a tabbed interface
Matrix Airfare Search: Lots of information, including a calendar of lowest fares
Kayak: Searches multiple sites at once
Momondo: A rival to Kayak, found by some seasoned travelers to produce better results
Booking Buddy: Searches multiple sites with a helpful tabbed interface
cfares: often has unique low rates
asap tickets service: phone-centered service–they have their own negotiated rates and frequently offer lowest fare guarantees. The agents are quite helpful.
Skyscanner
Wegolo: budget airline search
flycheapo: find budget airline routes within Europe
Ground Transport
For rail travel within the UK/Europe, book in advance or plan on paying at least twice as much.
In Transit
Bose® QuietComfort® 15 Acoustic Noise Cancelling® Headphones
Luggage
The International Traveller It-0-2 21″ Carry On Case weighs almost nothing, rolls well enough that it’s fun, fits within the most stringent overhead bin requirements (EasyJet’s), and it fits more than you’d think. I adore it.
Packing
Packwhiz: keep track of your packing lists for various types and lengths of trips. Add to them when you’ve forgotten something. When you’re unpacking and find items that have gone unused, consider taking them off.
Communication
Get one unlocked, GSM cell phone and a new SIM card for each country you plan to spend time in. You’ll have to keep track of the cards and renew them, but that’s better than carting around multiple phones. If you’re anything like me, you’ll eventually lose one of the chargers and be out another 20 pounds or euros.
Skype: almost goes without saying, but we’ll see what Microsoft does with it
Google Voice: If you’re living in the US, get this set up while you’re home so you can check your messages online while you’re abroad