Web-based learning has become an important way to enhance learning and teaching, offering many learning opportunities. A limitation of current Web-based learning is the restricted ability of students to personalize and annotate the learning materials. Providing personalized tools and analyzing some types of learning behavior, such as students' annotation, has attracted attention as a means to enhance Web-based learning. We describe a Web-based tool for creating and sharing annotations and investigate the effect on learning of its use with college students. First, an annotation tool was designed and implemented for the research. Second, learning support mechanisms, including full and group annotation sharing, were developed to promote students' motivation for annotation. Lastly, experiments with individual and shared annotation were conducted and the results show that the influence of annotation on learning performance becomes stronger with the use of sharing mechanisms. We conclude that there is value in further study of collaborative learning through shared annotation.