Toshiba 640GB 2.5" USB Portable Drive for $99.99 shipped.  Here is a screenshot for your reference.  This 2.5" portable drive does not require a separate power supply and is smaller than most external drives, making it more convenient to carry around.  At the time of this post, 92 out of 97 user reviews have given it 4/5 and 5/5 ratings.  Most places have it for about $119 or more.