Seafile is itself open-source file sync and share software; the Community Edition is free and AGPLv3-licensed, with clustering, S3 storage backends, SSO, and audit logs reserved for the paid Professional edition. So the comparison here is between Seafile's free tier limits and a different self-hosted platform's feature set, not open source versus proprietary.
If you want calendar, mail, and office document editing bundled in alongside file sync rather than file sync as a standalone tool, Nextcloud covers more of that ground in its free, open-source edition.
FAQ
Is there a free, open-source alternative to Seafile?
Seafile Community Edition is already free and open source. If you want a broader self-hosted suite instead, Nextcloud bundles file sync with calendar, mail, and office apps in one open-source platform.
What's the best open-source alternative to Seafile?
Nextcloud is the better pick if you want an all-in-one collaboration suite rather than Seafile's more focused file-sync-and-share approach.
What are the top open-source alternatives to Seafile?
Browse all open-source alternatives to Seafile above, or explore the full Collaboration category for more options.