ownCloud is itself open-source, self-hosted file sync and share software, with over 50 million users and a codebase you can run on your own server. So this is a comparison between two open-source self-hosted cloud platforms rather than escaping a closed vendor.
If you want a broader self-hosted suite that bundles calendar, mail, and office document editing alongside file sync rather than just file sync and share, Nextcloud — which forked from the same original codebase as ownCloud — covers more ground out of the box.
FAQ
Is there a free, open-source alternative to ownCloud?
ownCloud itself is already free and open source to self-host. If you want a broader self-hosted suite, Nextcloud — a fork of the original ownCloud project — adds calendar, mail, and office apps on top of file sync.
What's the best open-source alternative to ownCloud?
Nextcloud is the better pick if you want file sync plus calendar, mail, and collaborative office editing in one self-hosted platform rather than file sync alone.
What are the top open-source alternatives to ownCloud?
Browse all open-source alternatives to ownCloud above, or explore the full Collaboration category for more options.