I modified my standard, cheap Logitech optical mouse to include a small heating element. This really helps out on those cold nights when I am too cheap (or energy-conscious) to turn up the thermostat.
I opened the mouse, and located ground and +5V locations on the circuit board. I added two strings of 1/4w resistors that each have 4 10-ohm resistors in series. Each string is 40 ohms, so at 5V, this comes out to 5/40 = .125A or .625 W. The total heating power is 1.25 watt, and the total current (250mA) is well below the 500mA limit for each USB port.