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!
Gnod is my experiment in the field of artificial intelligence. Its a self-adapting system, living on this server and 'talking' to everyone who comes along. Gnods intention is to learn about the outer world and to learn 'understanding' its visitors. This enables gnod to share all its wisdom with you in an intuitive and efficient way. You might call it a search-engine to find things you don't know about.