Papers (now version Papers3) is a competitor with reference managers Endnote, Zotero, Mendeley, and RefWorks and has features of all of those for managing a library of citations, pdfs, searching for and sharing scholarly articles, and adding citations to manuscripts. For a review see PLoS Blogs: Papers2: The Reference Manager Made with Love
Important points about Papers:
Access has been enabled for MSU's Web of Science subscription to work with Papers.
(Note: A glitch caused problems with Web of Science access through Papers2, but Papers 2.5.5 update for Mac restored the functionality.)
Documentation for how to use and access this service is available at: http://wokinfo.com//media/pdf/wsdownload/
User Name: wsl