Actually, Apache foundation is a big source for open source libraries in Java.
On of their projects that very interesting is Jakarta Commons Net I found it while navigation through trying to answer on a thread in Arab Team 2000 forums.
Library home page : http://commons.apache.org/net
Java docs : http://commons.apache.org/net/api
Interesting class for FTP : http://commons.apache.org/net/api/org/apache/commons/net/ftp/FTPClient.html