Mendeley

http://www.mendeley.com/

"endeley Desktop is academic software that indexes and organizes all of your PDF documents and research papers into your own personal digital bibliography. It gathers document details from your PDFs allowing you to effortlessly search, organize and cite. It also looks up PubMed, CrossRef, DOIs and other related document details automatically. Drag and drop functionality makes populating the library quick and easy. The Web Importer allows you to quickly and easily import papers from resources such as Google Scholar, ACM, IEEE and many more at the click of a button."