EndnoteWeb, RefWorks, Connotea, CiteULike, Zotero, Mendeley. Nice summary of the state of the art by Martin Fenner. Conclusion - not much to choose in some ways - personal preference!
This is the winning entry into the Elsevier Article 2.0 Contest by. It demonstrates how scientific article publishing can be improved by applying Web 1.0, Web 2.0 and Web 3.0/Semantic Web approaches to add value to article content. The application enhances content navigation, allows commenting on specific paragraphs and features of images, and allows facts to be asserted about the article and its contents.