From the website:
Zotero [zoh-TAIR-oh] is a free, easy-to-use
Firefox extension to help you
collect, manage, and cite your research sources. It lives right where you do your work — in the
web browser itself. (beta version)
- Automatic capture of citation information from web pages
Storage of PDFs, files, images, links, and whole web pages
Flexible notetaking with autosave
Fast, as-you-type search through your materials
Playlist-like library organization, including saved searches (smart collections) and tags
Platform for new forms of digital research that can be extended with other web tools and services
Runs right in your web browser 
Formatted citation export (style list to grow rapidly)
Free and open source
Check it out at:
http://www.zotero.org/ and download
Firefox 2.0 here.
