Western Digital's My Passport line of drives are small, portable, and come in capacities ranging from 500GB all the way up to 2TB. All of the drives sport WD's "Smartware" automatic backup software, which allows you to back up your computer to your drive as soon as you connect it, or on regular intervals (although, we generally suggest you avoid any drive's built-in software and back up with something much better.) The My Passport line is most noted for being affordable and remarkably small—about the size of a pocket notebook—and being available in multiple colors and cases to fit your needs to both storage and style. The MyPassport line is also available with USB 3.0 (or 2.0), packs Western Digital's password-protection features (that keep the files on the drive from being accessed without authentication), and they're all powered by USB, so you don't need to drag an adapter along with you.
Those of you who nominated the My Passport line specifically praised its portability and its affordability, and one of you even mentioned liking WD's included security software—something we don't often hear when we're talking about external hard drives. If you want one, you can pick them up in your choice of size and color at Amazon, ranging from $60 for 500GB to $128 for 2TB.