Mendeley is freely available to everyone. Paid subscribers receive expanded storage. Web, desktop, and iOS interfaces are available.
Zotero is freely available to everyone. Paid subscribers receive expanded storage. Web and desktop interfaces are available.
Bibliographic management software helps you to keep track of books, journal articles, and other sources. It can automatically format your bibliography and in-text references as you write your paper. There are several options available; choose the one that suits you.