Zotero

I've blogged before about Citeulike.org an online reference manager as part of my Academic Ease series of posts. However an interesting alternative is a free Firefox extension called Zotero

Zotero is a mix between an online and offline citation management program. Because it is integrated into your web browser (Presuming you are using Firefox) it can be used online in much the same way as Citeulike. In fact it seems even more integrated with your browser, when it detects a paper that could be added to your library a little symbol pops up in your address bar (Like the RSS symbol, but a little notepad). Clicking this automatically adds the item to your library copying the citation details automatically and like citeulike allowing you to add tags. A particularly neat function is that from the table of contents pages for many journals if you click the notepad it will automatically give a list of the papers in that particular issue and ask you which ones you want to add to your library.

However where Zotero really shines is as an offline citation manager. While it is built into your web browser, all the information is stored locally on your machine meaning you do not need internet access to get to it. As such it effectively turns your web browser into a fairly well featured citation management program.
A particularly neat feature is that you can set it up to automatically save the PDF of any article which you add to your library. It saves these by paper and author name making them easy to find manually, and it maintains a link to the paper in Zotero so you can find the paper automatically as well. This means that Zotero is also a PDF manager, which if you have a burgeoning number of random PDF's on your computer as I do, this function alone makes Zotero a godsend.

There has been a plugin developed for MS Word so that Zotero can communicate with it directly and you can insert citations (in several different formats) directly from MS Word.

Perhaps the only downside is that it is currently only a Firefox extension so it is not available for those using Internet Explorer.