Western Digital Elements 640GB 2.5″ USB Portable Drive for $89.99 shipped. Add it to the cart to see the lowered price. Here is a screenshot for reference. It does not require a separate power supply and is smaller than most external drives, making it more convenient to carry around. Most places have this portable drive for about $99 or much more.
Originally posted earlier, it has been slightly revised and this is just a friendly reminder.