Connotea - citations meet web 2.0
Bibliographic information can be automatically fetched from a number of sources including PubMed, Nature, and it can handle DOIs. You can even bookmark posts from blogs, since Connotea makes use of the information within RSS or Atom feeds.
There is also an intersection between the Institutional Repository world and Connotea. I am involved in the creation of an institutional repository using EPrints and the opportunity to enable other researchers to link to our items is another plus point when selling the idea of open access deposition to our researchers. Many EPrints repositories have registered themselves with Connotea to enable citation information to be harvested.
I really like Connotea because it is a thoughtful implementation of Web 2.0 concepts and also, extremely useful and easy to use.
Take a look yourself!