CiteULike

CiteULike is a free service to help academics to share, store, and organise the academic papers they are reading. When you see a paper on the web that interests you, you can click one button and have it added to your personal library. CiteULike automatically extracts the citation details, so there's no need to type them in yourself. It all works from within your web browser. There's no need to install any special software. A similar site is Connotea, from Nature Publishing Group: www.connotea.org.

Features includes such as:-
1. The list of tags/keywords on the home page shows large and small tags; the larger ones are used more often
2. You can see other accounts who have saved the same articles as you; you can also see what other tags have been used for that article
3. If there is an article in another account that interests you, you can add it to your account as well.

For more detail visit: http://www.citeulike.org/