TagFlow
TagSpaces
allTags
Tabbles
Elyse
TMSU
TaggedFrog
HoudahSpot
Qiqqa
Mendeley
Zotero
JabRef
Citavi
Docear
Paperpile
BibDesk
TagFlowQiqqa is by far the strongest solution for research and managing pdf's. Zotero, named as a comparison is weak as it does not allow pdf;s to be loaded into its interface - you need to use a pdf reader, which defeats its whole purpose. With Qiqqa documents in a library can be searched together. It has excellent maps of your libraries. It is not prefect, having a few glitches, but still is the only one of its kind.
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.
JabRef - Graphical Java application for managing bibtex (. bib) databases.JabRef · JabRef Help · JabRef | Blog · OpenOffice/LibreOffice .