BibDesk - Mac only
Bookends - Mac only and Reference Miner (Bookends lite)
iTunes - method for utilizing iTunes as a PDF database manager
*Mendeley - Free web and desktop (Windows, Linux, Mac)
*Papers - Mac only
Quosa - limited management functionality and has it's strength in handling medical literature
Sente - Mac only
Yep - Mac only

* Best Bets

Managing your references at Dartmouth

BibTeX (http://www.bibtex.org/)

BibTeX resources from the Open Directory Project
Citeline an open source BibTeX tool to generate html files. Citeline works best with Firefox and there is a plugin for Zotero.

*citeulike

Connotea

*Endnote and Endnoteweb (free)

JabRef: a GUI for BibTeX (http://jabref.sourceforge.net/)
LaTeX Bibliography Styles Database - these are styles that can be inserted into your LaTeX document

*Refworks -  (Video tutorial)

*Zotero - Firefox extension

* Best Bets