CiteULike

Thanks to Pierre Lindenbaum, I discovered a free service similar to Connotea called CiteULike.

As noted by Pierre, both services offer advantages: for example, citeulike allows to store a personnal PDF copy of an article, while connotea can handle geospatial tags.

However, while Connotea is hosted by Nature Publishing Group, CiteULike is based on academic funds.