Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
TagSpaces - TagSpaces is an open source platform for personal data management. With TagSpaces you can manage and organize the files on your laptop, tablet or smart phone.
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
allTags - allTags is a free, tag based file management application.
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
Elyse - The free version allows up to 30 tags.