Geek Gifts to Avoid: USB Warmer Cooler
USB coolers and warmers are another "great idea, bad implementation" type of device, as Mark Kaelin wrote in this 2006 TechRepublic article. In his experience, the USB 5 volt connection did not provide enough heat to keep his beverage warm in a common ceramic coffee mug. The USB Beverage Cup Cooler and Warmer pictured (which is different than the one Mark tested) comes with some evidence that it may work better than Mark's experience, but I'm skeptical. If you like warm drinks, use a thermal mug, put enough of your beverage in for an hour or so max, and refill when it's empty.
Image courtesy VAT19.com.