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.