Justin Bryce, at A Dog Blog, has an interesting overview of the possible meanings of a dog's different barks. It sounds accurate to me, as it matches well with the barking that Penny does.
Since I heard Jan Fennell on the radio the other day I've been doing as she suggests and thanking Penny for barking at the front door. It's a bit of a bother, but when she barks to tell us something is happening outside, one of us gets up and goes to the door and looks out. We do what Jan suggested, and briefly say, "Thank you" and go away again. Penny will usually give one or maybe two more short barks and then stop.
We had been in the habit of going over when she barked anyway, unless it was absolutely too much trouble, as we figure that it's her job to tell us there is something going on that we should be aware of, and therefore it would be rude to ignore her. Very anthropomorphic, I realise, but it makes us feel good. Also, it's a good excuse to have a break from the computer or whatever we're doing.