(from Mendeley's Features page)
Mendeley is a London-based company that provides a free citation management software. Mendeley promotes collaborative work through an academic social network and weaves itself into the research process. As an integrated research tool that moves beyond simply organizng citations, Mendeley is considered the next generation of management tools.
Using Mendeley, you can:
Neat data visualization of Mendeley's development within the past year.
Interested in learning more about Mendeley? Here are some interesting articles for additional reading.