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!
WebCite® is an archiving system for webreferences (cited webpages and websites), which can be used by authors, editors, and publishers of scholarly papers and books, to ensure that cited webmaterial will remain available to readers in the future. Authors