Western Digital Caviar 640GB 7200RPM 16MB Cache Internal Hard Drive for $57.89 shipped. Here is a screenshot for your reference. This drive uses a high-density 2-platter design for better performance while reducing power consumption. Note that it comes in OEM packaging, which means no pretty retail box and marketing materials but is otherwise identical.
It is now out of stock. Congratulations to those who were able to get in on it!