Zotero - a research library

Zotero describes itself as "a free, easy-to-use tool to help you collect, organize, cite, and share your research sources."  

Zotero collects all your research in a single, searchable interface. You can add PDFs, images, audio and video files, snapshots of web pages etc.

Imagine this is most useful for researchers and academics, whether working alone or collaborating with others.