Coolant leaking from the drivers side of the engine? Why?

In the past few days I have noticed that when I park my car and come back to it a couple of hours later what looks to be coolant has leaked from the drivers side of the engine, out underneath the car. (I'm pretty sure its coolant because the bottle that hold the antifreeze etc is constantly emptied after the leak). Has anyone got any ideas of what this problem is and exactly how much it will cost to put right? I have a ford puma from about 2000/01.