FileZilla is one great open source FTP client that -- up until now -- was available only for Windows. Version 3 is a ground-up rewrite that makes the application available for the first time on Linux, too.

You can download the latest FileZilla release either from filezilla-project.org or through the project's SourceForge.net page. In both places, you will find the older FileZilla 2.x series for Windows only, and the 3.0 series for Linux, Mac OS X, FreeBSD, and Windows.