Zotero is free and open source, meaning its source code is open and continually improved by an active user base. Zotero is compatible with Windows, macOS, and Linux. It is compatible with the Chrome, Safari, Firefox, and Opera browsers, and has plugins for Microsoft Word and LibreOffice.
Installing Zotero is easy:
Below is an example of the Zotero client. Notice that it is divided into three segments--one for collections (far left), one that lists items within a collection (middle), and one that presents an individual item's metadata (far right).