The FileZilla Client not only supports FTP, but also FTP over TLS (FTPS) and SFTP. It is open source software distributed free of charge under the terms of the GNU General Public License.
We are also offering FileZilla Pro, with additional protocol support for WebDAV, Amazon S3, Backblaze B2, Dropbox, Microsoft OneDrive, Google Drive, Microsoft Azure Blob and File Storage, and Google Cloud Storage.
Last but not least, FileZilla Server is a free open source FTP and FTPS Server.
Support is available through our forums, the wiki and the bug and feature request trackers.
In addition, you will find documentation on how to compile FileZilla and nightly builds for multiple platforms in the development section.