I've never understood usb powered coasters that are supposed to keep your cuppa hot.
If your cuppa goes cold in the time it takes you to drink it, drink it faster. Coffee is medicinal, and you can never have enough tea.
However, with this inspiration, I can take a stab at another problem.
Why do fingers freeze when in contact with computer mice?
It seems I'm a bit late to this party, with commercial units for about $25, but it's so easy to do yourself with the seemingly endless supply of discarded mice.
With this in mind, I present my USB heated optical mouse.
Step 1: Power
So you want to make things hot. Power is the most important thing.
If you place a hot cup of tea on a set of scales and allow it to cool, it looses about 25 watts of power by evaporation alone.
(measurable by mass of water evaporated per unit time * latent heat of vaporisation)
My soldering iron would have trouble keeping up with that.
USB specifications allow you to have 2.5 Watts total. (5V * 500mA) Feeble. However, this can get comfortably warm.
To maximise power draw without your computer making a fuss, you can place 10 ohms accross the power rails in your USB device.
Two 22 Ohm resistors in parallel should do while leaving a bit of room for the original circuit.
In this case, I need a nice distributed heat so I'm going for two 10W wire-wound resistors simply for the size. Distributing the heat means you don't melt the plastic.