Mendeley is a social network-based citation manager that collects the references of its users into a big searchable database. It also allows users to create groups and, sometimes, trade between themselves PDFs of the articles they use. PDFs can be annotated in Mendeley Desktop.
Mendeley was created by three Ph.D students in 2007, and the company was founded in 2008. It was later sold to Elsevier in 2013 (an interesting commentary on this sale can be found at the New Yorker: "When the Rebel Alliance Sells Out," by David Dobbs, April 12 2013).
The basic storage plan (2GB) is free, but more storage is available for a fee:
(Source: Mendeley Premium Packages)
Citation styles are written in the XML-based Citation Style Language (CSL). Citation management tools all come preset with many of the major styles (APA, MLA, Chicago, AMA, etc.) but Mendeley can also import new CSL files to format your citations in the style of whatever journal or publisher with whom you might be working. Styles can be downloaded within Mendeley Desktop by going to Citation Styles under the View tab, and then click on More Styles. The screen shown here will let you search for the style you need and install it that way, or download via the source URL from one of the libraries below. It will then appear in the citation style options when you export your items as a bibliography.
Signing up with Mendeley and importing or linking yourself to your papers can be one method of tracking your impact as a scholar. Mendeley's catalog of papers (online, not in Desktop) shows who has read your paper, what discipline they are in, their academic status, and their location.