How old is the dog? Could it be he/she is going through their adolecent stage? I hear puppys get very bad during this time!
My puppy used to not bark AT ALL. She would even sleep through the night without a peep. After she was a few months old (I think she started doing it at around 4 mos) she would bark for EVERYTHING. She would bark for HOURS at night even though I would get up to see if she had to go potty.
She mainly barked when she was in her crate and I was in another room. She HATED it. I think she just didn't like not being included in stuff giong on or she was scared. Recently though, she has gotten over it and will sit or lay nicely in her crate until she is let out. One thing I did was put her in the crate, and if she was a good puppy, I would give her a treat and let her play. I did that several times (mainly on the weekends) and now she will happily sit in her crate until I let her out. Now if she wants out she will whimper to let me know she's still waiting to be let out
My neighbors downstairs have a dog too, and I noticed sometimes they would bark to each other (at least it sounded that way... one would bark, and then a few moments later the other one would bark), so if there are other dogs around, maybe your dog is talking to them?
I couldn't bear the thought of putting a shock collar on my dog
I also second the Kong Treat balls!!! My dog LOVES hers and it will keep her busy forever.
PetStages also has a little treat-holder I have for my dog. Basically it is a green soft plastic "cage" that you can put big treats in. I love them b/c they keep my dog busy and my dog likes them b/c she spends a long time trying to figure out how she is going to get the treats out