The AC drain pipe! All cars with A/C have a drain pipe to dump the condensate. A/C removes excess humidity from the air, this is condensed and channeled away by pipe. Look under any car with A/C and see the same. The outflow continues for a while after you switch the engine off.
Unless you have made a point of turning the A/C off then then it could well be running even when not needed to cool the car. Check whether your car has an economy mode which should enable you to switch off the heat pump.
It could just be water draining from the gutter under the windscreen.
Whoops, I didn't mean to start an argument. My car is a two year old Honda Accord, is that info. any help in answering my query? The puddle is there even though I have not switched on the A/C, but I must admit I hadn't considered the possibility of rainwater draining off the windscreen.
Since posting my last answer I have realised that the puddle is towards the front of the car and I would imagine that a rainwater drain would discharge directly below the windscreen. I rather feel that the rainwater theory can be discarded.