”Zotero is a free and open-source reference management software to manage bibliographic data and related research materials” — Zotero, 2019
Compared to our current standard software Endnote, Zotero offers offers additional features like cloud-storage or automated access through API (e.g. the R package RefManageR is able to connect to a Zotero database using the function ReadZotero().
For a full comparison of the features of different reference management software available see Wikipedia, 2019.