CiteULike

http://www.citeulike.org/ 
The website is sponsored by the publisher Springer (http://en.wikipedia.org/wiki/Springer_Science%2BBusiness_Media).

http://en.wikipedia.org/wiki/Comparison_of_reference_management_software

Import file formats: RIS, bibTex
It also allows to import into the free Zotero bibliography extension of Firefox.
Export file formats: RIS, bibTex, COinS, Delicious

Reference list file formats: HTML, RTF, PDF, RSS

No word processor integration.

Good database connectivity

CiteULike is based on the principle of social bookmarking.

When browsing issues of research journals, small scripts stored in bookmarks (bookmarklets) allow one to import articles from repositories like PubMed. Then the system attempts to determine the article metadata automatically. Users can organize their libraries with freely chosen tags.
A folksonomy is a system of classification derived from the practice and method of collaboratively creating and managing tags to annotate and categorize content; this practice is also known as collaborative tagging, social classification, social indexing, and social tagging.

----------------français
CiteULike est une application web de gestion bibliographique, dédiée principalement à l'organisation des publications scientifiques.