Zotero

Zotero currently allows users to export references in three citation styles: APA, MLA, Chicago

http://www.zotero.org/translators/
http://www.zotero.org/documentation/compatible_standards_and_software

Endnote

del.icio.us

CiteULike

Connotea

social bookmarking sites like del.icio.us and reference management sites like Connotea