We have a miniature dachshund who is getting on in years (nearly 12 years old), but he is the best guard dog I have ever owned. He will be asleep in the back bedroom, and come to alert as soon as someone begins to pull into our driveway.
He can even recognize cars - if it's my fiancee or one of my friends that comes over on a very regular basis, he will just alert and keep his ears cocked (or in my fiancee's case, take off full-tilt wagging tail and barking like a madman to greet her). If it's someone he doesn't know, he will take off to the front door to investigate. It's honestly really impressive, he recognizes the distinct sounds of their cars. His bark will wake up everyone in the house, in any room in the house. It's scary as crap to wake up to that.
Our cat is a good watchcat too, he'll take off 100mph for cover if he sees or hears anything that's out of the ordinary. Some of my friends didn't know I had a cat for a while, he hides whenever anyone he doesn't know is nearby. The cat is neurotic. He's hidden under our bed for days before when we had guests over. If he needs food or water, he runs out, drinks and eats as fast as he can, and takes off back under the bed.
It's pretty accepting of us, though
The lab/pitbull mix we have, on the otherhand, is completely silent. He doesn't bark unless he's playing with me, or someone's about to be taken down. He follows the dachshund's lead, too. If the dachshund freaks, he goes on alert. Otherwise he lazes around and acts like a 90lb footrest
. He's also completely mellow and gentle compared to the dachshund, but when he gets serious, he's incredibly strong and I would seriously hate to have him coming at me. I'm a big ol' 6'4" 320lb boy, but he can knock me flat on my seat.