Zotero is an amazing new bibliographic reference manager. Calling it an "extension" doesn't do it justice. It behaves like a full-fledged application within Firefox. If you're a student, academician, researcher or writer, you really should check this out. If you've been saving for Endnote or RefWorks, you can probably spend that money on something else.
From the web site at http://www.zotero.org/
"Zotero is a free, easy-to-use research tool that helps you gather and organize resources (whether bibliography or the full text of articles), and then lets you to annotate, organize, and share the results of your research. It includes the best parts of older reference manager software (like EndNote)—the ability to store full reference information in author, title, and publication fields and to export that as formatted references—and the best parts of modern software such as del.icio.us or iTunes, like the ability to sort, tag, and search in advanced ways. Using its unique ability to sense when you are viewing a book, article, or other resource on the web, Zotero will—on many major research sites—find and automatically save the full reference information for you in the correct fields."
Note that Zotero requires Firefox 2.0.