Nextcloud is a suite of client-server software for creating and using file hosting services. It is functionally similar to Dropbox, although Nextcloud is free and open-source, allowing anyone to install and operate it on a private server.

In contrast to proprietary services like Dropbox, the open architecture allows adding additional functionality to the server in form of applications and enables the user to have full control of their data.

The original ownCloud developer Frank Karlitschek forked ownCloud and created Nextcloud, which continues to be actively developed by Frank and other members of the original ownCloud team.

Read more:
https://nextcloud.com/