If you write in Overleaf or LaTeX, JabRef is a powerful, open-source manager for BibTeX files. It’s completely free but lacks built-in cloud sync (use Dropbox or GitHub for that).
Zotero is a free, open-source reference management tool. It is widely considered the best free alternative to EndNote. endnote x6 product key free free
Before you give up on EndNote, check with your university library. Many institutions pay for a site license for EndNote that allows students and faculty to download the current version of EndNote for free. This is the only safe way to get a "free" EndNote product key. If you write in Overleaf or LaTeX, JabRef
Mendeley offers a generous 2 GB free cloud storage and strong PDF annotation tools. It’s ideal for researchers who work in teams or need to highlight and comment on PDFs. It is widely considered the best free alternative to EndNote
Limitations: The desktop app is being phased down in favor of the web version, so some features may change.