Zotero
Zotero. Free, easy-to-use research tool. Gathers and organizes full text articles, web pages, citations, or bibliographies. Lets you annotate
Zotero: Firefox(2)-based citation manager
I tried using Zotero on scientific journal sites such as PLOS, Nature,Science, Elsevier's Science Direct and it works perfectly. I have tried all their claimed features and they work seamlessly. When compared to commercial equivalents like Endnote and R
Zotero |:| Quick Start Guide
Zotero - The Next-Generation Research Tool » Compatible sites
Zotero Forums - Categories