E-negotiations are more and more popular as the use of Internet grows. The need for Negotiation Support Systems (NSS) is thus more and more obvious. Various NSS have been designed to help developers in building their negotiation applications, such as Inspire, the SilkRoad project, Magnet or the GNP platform. These NSSs are often dedicated to e-commerce, which is a subset of existing negotiation applications. That's why we have designed Genca, our Generic Negotiation of Contracts API, that aims to facilitate the design and implementation of contract-based negotiation applications, not only in the e-commerce field but also in non-commercial purposes. In this paper, we show that Genca is the best suited model for developing a negotiation application. To do so, we propose a set of criteria to classify the NSSs and compare them.