From the website:
"Zotero is a powerful, easy-to-use research tool that helps you gather, organize, and analyze sources and then share the results of your research."
It can be used as a browser plugin, but the standalone version has some advantages. Zotero is open source. Unfortunately it is usually installed as a binary (https://download.zotero.org/standalone/18.104.22.168/Zotero-22.214.171.124_linux-x86_64.tar.bz2). The sources are kept on GitHub (https://github.com/zotero).