Similar to traditional citation management programs, like Endnote, Mendeley helps collect citation information; keeps your research in a central, organized place; and makes it easier to insert references into your academic writing. In addition to these so-called "citation management" tasks, Mendeley also has drag-and-drop functionality for importing PDF files into your Mendeley database. That means that if you've already collected many (or a few) articles, and you're looking for a way to organize your efforts, Mendeley can help.
The same is true if you've just found and downloaded a new PDF file and want to be sure not to lose it in your own maze of a computer. Mendeley also has workflow tools such as annotations, tagging and full-text PDF searching, which allow searching and categorizing to facilitate your thinking and writing around these files. It also allows users to share citation information and PDF libraries between collaborators. (Something like: "You uploaded and are reviewing this paper...other people who also uploaded it are reading these other papers...")
It's free to anyone with internet access with additional features (ie additional space) for a fee.
Troubleshooting & help
For assistance please see the Citation Management Self Service guide.