Many of the possibilities of Web-based education are still unexplored. It seems that novel ways of thinking about both learning and technology are needed to get beyond the limitations of the traditional classroom setting. In this paper we introduce EDUCOSM, which focusses on the possibilities of collaboration and the open-ended nature of the Web. Its main features include sharing and annotation of arbitrary Web-pages, and an adaptive desktop for accessing the evolving contents of the system. EDUCOSM has been used in a real Web-based course, and the experiences are discussed along with a description of the tool. Although the approach requires both the teacher and the students to re- think their roles, the feedback received so far has been encouraging.