Zotero is a free, easy to use bibliographic manager that collects, organizes, and helps you cite your research. It has desktop, online, and browser-integrated versions. It comes with the most heavily used citation styles already available, but you can download many more.
You can attach PDFs, take website snapshots, organize your sources into folders, and create bibliographies. It integrates with Word, Google Docs, LibreOffice, and Open Office.
Zotero will also index the full text of your documents, allowing you to find exactly what you need.
You can organize your citations into Collections so that all of the information surrounding one topic is gathered together. Collection names can reflect a specific project or a class title or number similar to a folder on your computer. A reference can be in more than one Collection at at time.
To create a collection: