Rhakios wrote:P=VI (IIRC)
So for USB 2.0 that's max 5.25V and 500mA, which gives 2.76W. I'm not sure how warm 2.76W will keep your cup of coffee.
Whatever Ohm's Law says about it, someone has not only invented, but marketed a USB cup warmer.
Edit: here's another one:
It seems to be MacOS and AmigaOS compatible, but it doesn't say whether it will work with Linux.
Edit2: and another one:
The world seems to be awash with them.