On Jan 27, 2013 Taylor Swift announced on her YouTube channel that she has signed a deal with Diet Coke.  This likely has nothing to do with her ex-boyfriend Harry Styles and his band signing a deal with Pepsi, as these deals take time to go through, and she was probably still with him when the negotiations started. (to clarify any assumption some make in the YouTube video comments).
Taylor has advertised for a Sony camera. She was shown in a commercial for the camera.