Paperless-Home - Find your documents when you need them most. Paperless manages them so you never have to dig through piles of paper or chaotic folders on your PC again.
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Teedy - Teedy is a lightweight document management system packed with all the features you can expect from big expensive solutions but still easy to use.
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Paperless - Developers and publishers of Mac, Win, and iOS productivity, home and office and writing software such as MacGourmet, Paperless, MacJournal and many more. Based in MN.
Qiqqa - Qiqqa is a free research and reference management software. It can be used in many organizational projects from the academic to the personal to the business endeavor. Read more about Qiqqa.