As the other posters have mentioned, fixing your dog if you haven't already is a solution that you definitely want to look at. It will definitely make your dog less aggressive by way of removing testosterone from his system. Once neutered, a dog's aggressive tendencies will diminish quite a bit and he will hopefully be less problematic around strangers.
That being said, dogs will often take cues from their owners. The way that you carry yourself when you're around other people can speak volumes to a dog. If you look apprehensive, or withdrawn while speaking to a stranger the dog could misinterpret it as the stranger causing you to be uncomfortable. (Whether or not this is the case is up to you, but it might be what the dog sees.) The dog would then protect you as a member of its pack by essentially telling the stranger to "back off".
Check your dog, but also check that your posturing and attitude isn't being reflected by your dog.
