Productive Scholar

CiteULike is a free online service that lets you manage your scholarly papers. The idea is simple: you create an account and then you can add references in your library. It has a nice features set which includes:

• Export all your references to BibTex and EndNote.
• Integrates nicely with many online database (see here for complete list). This is actually very nice; when you add a paper from one of those database (using the url), CiteULike imports all the bibliographic information plus the abstract. For some databases, it will also add a link to the pdf file of the full article (of course, you need access to the database for that).
• Lets you upload your own pdf files so you can access them from anywhere.
• Create groups so you can share papers with your colleagues or other people interested by a topic.
• Bookmarklet for adding papers quickly (a bookmark that contains javascript). When you are on an article page (like PubMed or JStor), you can just click that bookmark to add the reference to your library.

Those are just the features I find the most useful, there are plenty more. Pretty good for a free site.