The Universal Java FTP Client -- Free for Non-Commerical Use

Java FTP or JFTP is a GUI-based FTP client software. It can transfer files between various operating systems. It also incorporates Secured Sockets Layer (SSL) to securely transfer sensitive data.

JFTP is written entirely in Java, which means it can run on various platforms such as Windows, Mac OS X, Linux, Solaris, HP-UX, OS/2, UNIX, or any other platform with a compatible JVM (Java Virtual Machine).

I am a personal user of this product and it's a convenient tool, especially the multiple FTP sessions. Of course, I'm using the non-commercial version. However, the commerical version can be obtained for a nominal fee.

One last thing, you can run a Java Applet from their site. I would recommend downloading a copy because it seems to be faster. Btw, this was not a paid post.

Other FTP Clients/Servers
ncFTP
ProFTPD