Zotero is a free bibliographic software extension for Firefox browser users. Features include automatic capture of information from websites, formatted citation export, integration with Microsoft Word.
BibSonomy is a free social bookmarking service enabling exporting in a variety of formats and feeds.
JabRef is an open source bibliography reference manager that interfaces with Bibsonomy but can also be used stand-alone.
Mendeley is a free reference manager and academic social network that can help you organize your research, collaborate with others online, and discover the latest research. It is primarily aimed at academic staff.
CiteULike is a free social bookmarking service to help academics to share, store, and organise the academic papers they are reading. It specialises in academic papers.
CiteFast is a quick online citation generator. Select your style, type in the bibliographis details, and paste the generated citation.