Qiqqa 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.
DocFetcher - DocFetcher is a portable German/English open source desktop search application.
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Recoll - Recoll is a desktop full-text search tool. Recoll finds keywords inside documents as well as file names.
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Advanced Recent Access - Advanced Recent Access is designed to see more properties of your recent used resources (files and directories). Key features: can see more properties of the recent resources, such as path, size, type, date modified and date created. And more.
JabRef - Graphical Java application for managing bibtex (. bib) databases.JabRef · JabRef Help · JabRef | Blog · OpenOffice/LibreOffice .