Free reference manager. Automatically generate bibliographies; import and annotate PDFs; collaborate easily, find relevant papers based on what you’re reading, access your papers from anywhere online; iPhone app

Free software allows you to store and organize citations as you find them. Create groups, add a wide variety of items, or store a snapshot of a website.

Zotero 5.0

These librarians listed most of the features that you would want to know about (and they've listed more features than other charts I've seen).