Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
PDF Stacks - PDF Stacks software program helps you organize and manage PDF files, papers and documents. Easily search and read your PDF documents on a PC and MAC
EndNote - Accelerate Your Research. Save time, stay organized, collaborate with colleagues and get published with EndNote 20.
BibDesk - BibDesk is an organizational software created to help you edit and manage your bibliography. It keeps track of your bibliographic information as well as said information's associated web links and files. Read more about BibDesk.
Docear - Docear