Interestingly enough, my Lab makes a pretty good watchdog. If she hears any noises in the yard she goes on alert, and if somebody tries the door or rings the bell she sounds like a VERY dangerous dog - a big growl that comes all the way from her belly and thunderous deep barks (completely different from the high-pitched bark she has when she's after a squirrel.)
Of course, once the door opens the only thing she's likely to do to you is get your face all wet jumping up and licking you . . . but she sounds very convincing.
My little Standard Schnauzer Emma, 40 pounds, does an excellent job of warning off intruders, postmen and pizza delivery boys.