The aim of this thesis is to examine collaborative tagging as a potential asset to libraries by a review of 10 social bookmarking sites and their features, and a discussion of these features’ possible use when the environment changes from the web to a library. Previous research on collaborative tagging is presented, as well as overviews of social bookmarking, folksonomies and the concept of Web 2.0 and Library 2.0. The study itself is presented as a walkthrough of common features and how they work, followed by a schematic overview of which sites have which features. Finally each site is presented in a more detailed manner. An example of social bookmarking within a library, PennTags, is then also described. The results show that there are great differences to be found among the social bookmarking sites, in both features and in which tools they offer. As some features have become more common since previous similar studies, there are also one or two unique features for almost every site. Many of the younger sites have adopted the more social approach instead of being mainly an organizational tool, with an emphasis on features like groups and lists of friends. The analysis uses previous research and the results combined to answer which features could be suitable for a library environment, and how to use them. The conclusion is that many features can be considered useful in a library portal, although not all, and that the motives and preferences of each library affect which features are suitable. / Uppsatsnivå: D