A 44 watt non-thermostatic de-icer. The heating element is laminated between foil. @ $13-14.00 http://www.avianaquatics.com/winterwater.html it sorta looks like this

a small maxi 1000 pump usually used for aquariums- powerhead but it has a "nose" which allows a small hose to attach (use a screw clamp to fasten)..... found at that pet place, etc. online discount places.  This can be put into a little bucket filter or wrapped in course filter material to prevent it getting clogged.

1. plug in a multi-plug extension cord, put a zip lock plastic bag on the multi-end where everything is going to be plugged in.
2. Set a flat rock just under the surface of the water. I have a flat rock sitting on top of an overturned plastic plant pot.
3.  set the flat heater on the rock so it is also just under the surface.
4.  set the pump so water from the tubing hose flows over the flat heater.  rocks may be needed to hold the tubing in place.
5. plug everything in and zip the bag shut to keep the connections dry.  I cover the bag with another flat rock, but snow will cover it nicely.
6.  while this is good for a couple fish in a small pond dropping an airstone or even two into the water next to the flat stone is a great idea.  it will oxygenate the water nicely, also help remove toxic gases.  the cleaner the bottom of the pond going into winter, the better.  netting small ponds in fall to keep leaves out makes cleanup a snap.

the added benefit of this hole in the ice is birds WILL find it all winter.  the downside is they may also foul the water.