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.

Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.

allTags - allTags is a free, tag based file management application.

Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.

Tabbles - Tabbles use tags to organize and find files along with your colleagues.

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.