coolant leak

At a stoplight, the driver next to me pointed out that my car was leaking a lot of coolant.  I  decided to drive the few blocks to my home since the temperature gage  seemed to be okay.  When I got home I did see a healthy stream of antifreeze coming from underneath my car.  A couple of hours later I checked the overflow bottle, and it was empty,  But the radiator still had coolant, and I was not able to add any water.  Also, I loosened the lower radiator hose and antifreeze came pouring out indicating a full radiator. Before I have the car towed to a mechanic, I would like to know what the possible causes of the leak could be.  If it's not a major problem, I would try to fix it myself.