The Bark Repeller from Pet Parade is awesome!
There's a thread somewhere on here about it and it nearly worked for everybody. My dad just received it from Amazon.com and he has 2 barkers, so when I bring Jackson over, he also barks more. I was just over there yesterday, and it's soooo much more peaceful. It just emits a sound that they don't like, and humans can't hear it (unless you put it right up to your ear). Use a word like 'quiet' along with it, and soon you won't need the remote. My dads neighborhood has a lot of loose dogs roaming around so that was a huge reason for ours barking.... my step-mom said she brought it on her walk with her and her dogs usually bark like crazy at other dogs on walks and all she did was hit the noise and they stopped!