There are two main ways that your car can leak coolant. The first is a faulty head gasket, which you don't want... The second is a through a small "weep" hole in the bottom of the water pump, which means that your pump needs to be replaced. Even though the pump is leaking, coolant may or may not collect under the car. Keep an eye on these two places when checking your system...
Also add the coolant (engine) temperature sensor to your grocery list of items that may cause excessive fuel consumption.
Hope that helps!