1. A computer-implemented method for product recommendation, the method comprising: by at least one computer system, determining a similarity percentage between a first list of products and each of a plurality of a second list of products, wherein each product that appears on at least one second list and does not appear on the first list is a candidate product; for each candidate product, determining the sum, across the second lists, of each instance of the candidate product weighted by the determined similarity percentage for the second list upon which the instance of the candidate product appears; and recommending candidate products in order of determined sum of the candidate product.