Refined products
Cutting drugs with things like glucose, sucrose, mannitol etc is safe (although I'd prefer if people didn't!), as they are carbohydrates, and these do not provoke an immune response. You could even cut drugs with amino acids (such as phenylalanine) safely, but once you use compounds that have two or more amino acids joined together by an amide bond, you have a simple peptide, and then you have something potentially dangerous (peptides are effectivly short chain proteins - I don't think there is any hard and fast rule as to when peptides get called proteins, but as a rule of thumb, a few amino acids form a peptide, lots form a protein).
I've heard stories about people using all sorts of crap to cut drugs with, but I've never heard of anybody using something like gelatin powder (even though finely ground up it has the same tan colour as some smack I've seen people use), as it's a good way to either kill or seriously damage the person using it (if they inject). All the things like glucose for food use are highly refined (read: purified), so there are no proteins present.
Baking powder is a mixture of sodium bicarbonate and tartaric acid. Add it to water and it will fizz first (releasing carbon dioxide), leaving a soln of sodium tartrate. In very small amounts, it wont do anything, but large amounts will alter the pH of plasma, which can cause an endless number of problems, some very serious.