Seagate GoFlex Slim 320GB The Seagate is a 9mm beauty. Ultra thin and sleek. Comes with USB 3.0 port on a removable connector port, that can be switch for a firewire 800 or even a thunderbolt one. That's future proofing guaranteed!
Iomega Prestige 500GB The metallic look of the veteran company Iomega, is definitely a looker. It comes supplied with a double headed USB cable for easy connection and it draws power from two USB ports, so you won't need a power brick.
Lacie Rikiki Go 1TB The Lacie comes with a built in cable that could be a blessing and a curse. It may be too short for some computer's set ups and the fact that is super short, means that eventually it will break (and they do break!). So bring an additional USB cable, because, this one is removable (thank God) 1TB is more than enough these days (at least for now) but the drives comes NTFS out of the box, so you may need to re-format it before starting to enjoy transferring data.
Freecom Mobile Mg 750GB With a magnesium body, what's not to like? This German pocket size hard drive is built to last several generations. It has a decent capacity and is extremely lightweight. Comes pre formatted for Mac OS and even has Firewire 800 connections as well as USB.
Other skinny alternatives: