Chocalate, coffee, macadamia nuts (?) and raisins are all supposed to be bad for a dog (though I've never heard of a dog suffering from eating any of them).
This page offers details:
http://www.petalia.com.au/Templates/...s&story_no=257
Peanut butter, on the other hand. As long as it's only a little bit, it's just plain fun.
One thing to consider - the more you feed a dog human food, the more he becomes confused about what he can and cannot eat in the kitchen. Eventually, it will make leaving food out (before a party, etc.) more dangerous.