In this paper we report an extensive analysis of the different business models for auctions found on the Web. Three hundred websites randomly selected from the main search engines are analyzed, classified, commented and compared in terms of their basic parameters. Departing from a theoretical review of the classical auction theory applied to the online world, twelve business models are defined and analyzed. The confusing terminology regarding the new online auction models is also conveniently organized. The results show that the new possibilities brought by the popularity of the Internet and the World Wide Web are making possible emergent business models in both the business-to-consumer and the business-to-business arena.