If the weather gets cold (30-50 degrees), just moving the water on a continuous basis will eliminate the opportunity for freezing and also limit the amount of mosquitos that lay eggs.  An opportune product is the Solar Bird Hanging Birdbath pictured to the right by Zoobox. 

Although it is not touted as a heater, it will do the job of eliminating ice in milder climates.

If you live in a harsher climate, a bird bath de-icer or heater is more appropriate.

Many bird bath de-icers cost less to run than the average night light. Low wattage is = low operating costs, so when you look for a bird bath de-icer – check that the wattage is low to minimize energy consumption and overall cost to you.   In addition, look for a de-icer that is thermostatically controlled .  This is important so that it doesn’t run and use up energy when it doesn’t have too.

The bottom line, a solar powered bird bath solution is your best and cheapest solution, unless you live in an area that has harsh winters.  If that is the case, look for low wattage and a thermostatically controlled bird bath de-icer.