As a freelancer, do I charge agencies (design firms, advertising / marketing firms,) tax on a final invoice?

I will try to answer your question, but I am not by any means a CPA, so what I tell you should be checked out directly with a professional. Usually, as a freelancer, you only charge tax on the final product. If an agency has contracted you to do a project, they will be the ones completing the final product for the client. Thus, charging the client tax. In order for the agency to be tax exempt, they must provide a Tax ID number and fill out a tax exemption form. Check with your state for state tax regulations and requirements.