Zotero (pronounced "zoh-TAIR-oh") is a tool for collecting, managing, and citing research sources. It's easy to use, connects to your web browser where you do your work, and best of all, it's free and open source!
Zotero allows you to attach PDFs, notes, and images to your citations, organize them into collections for different projects, and create bibliographies.
It automatically updates itself periodically to work with new online sources and new bibliographic styles.
There are plenty of other options out there. Some of the more notable ones include:
For more, check out this handy Citation Managers Comparison Chart from the University of Wisconsin libraries.