Seagate FreeAgent Go 640GB 2.5" USB Portable Drive for $119.00 shipped  Here is a screenshot for your reference.  It is USB powered and does not require an external power supply.  It is smaller than most external hard drives, making it more convenient to carry around.  Note that 640GB is currently the largest capacity for a 2.5" portable USB-powered external drive.  Most places sell this portable 2.5" external drive for over $145.