Zotero is a free, open source desktop application that allows you to collect, organize, cite, and share research. It works on Windows, Mac, and Linux operating systems. This guide has information on how to get started with using Zotero, from installing the application and its plugins, to adding sources to your library, to generating in-text citations and reference lists.
Zotero 6 was recently released with some exciting new features, including integrated PDF annotation.