Zotero (pronounced "zoh-TAIR-oh") is a free application that collects, manages, and cites research sources. It's easy to use, connects with your web browser to download sources.
You are given 300 MB of storage space for free from Zotero; additional storage is available for reasonable fees.
Zotero allows you to attach PDFs, notes and images to your citations, organize them into collections for different projects, and create bibliographies.