The seeds in grapes are poisonous to dogs. As for chocolate, baking chocolate is the worst, then dark chocolate. Milk chocolate is not so bad depending on the amount of chocolate and the size of the dog.

Here is an excellent link that talks about the toxicity of chocolate and other things.

http://www.envtox.ucdavis.edu/cehs/TOXINS/dogs.htm

Here is a rather complete list:

http://www.hsus.org/pets/pet_care/pr...s_to_pets.html