The name is awful, but CiteULike looks like a potentially useful tool. Sign up, then use a bookmarklet to post citations of articles from JSTOR (for example; other sources includes PubMed) to your CiteULike list. (del.icio.us works the same way, for non-academic links.)
CiteULike tracks popular links and categories (it’s network-heavy at the moment), and promises more features in the future: Importing (and exporting?) citations, support for other databases, and so on. Depending on how it develops, CiteULike could be a neat companion, for some uses, to other bibliographic software.