Free, open-source substitutes for Mendeley? Mendeley is (mainly) a proprietary social network to share basic citation data and research papers. see also

Are there any free and open source substitutes for this service? 

A technical possibility: users could collect BibTeX data and a hash and share this information. 
It would be very useful to have such a free service because every journal provides the citation data in a different format. That includes false field entries, broken files and hidden download buttons on the website.
The only alternative that comes to my mind is Zotero:


It is open-source, 
it comes as a standalone application or as a web-based version with Firefox, Chrome and Safari connectors, 
it integrates with Word or OpenOffice, 
it syncs with the Zotero server, 
it has BibTeX export,
and more.


The Zotero standalone client is cross-platform and open-source (AGPL licence), and it can be run on its own or synchronized with the web version. The web service is free to use up to a fixed storage quota, with paid storage available, and there is an open-source implementation of the dataserver available if you want to roll your own. The local client stores its data in SQLite format so in principle your data is not locked in, but the database is relatively hard to trawl externally; however, since the client is open source there are relatively few future-proofing concerns.