Robust and proven - in use in tens of thousands of applications worldwide.
Edit By BS Editor: Free Java FTP library gives Java developers:

* Extensive FTP functionality.
* Robust and proven - in use in tens of thousands of applications worldwide.
* Java source-code included!
* Open source and free for commercial use (LGPL).
* Downloaded over 200,000 times!
* Widely used throughout the world in thousands of projects.
* Full source code is provided.
* Passive and active modes are supported (PASV and PORT).
* Resuming of interrupted binary transfers supported.
* Support for many FTP extensions such as MDTM, MLST, and SIZE.
* Now compatible with Java 1.1.x, so can be used in applets and many handheld devices.
* Under the LGPL, so it can be embedded in commercial applications.
* Unrestricted licences are also available for low cost.
* Mature and stable CodeBase.
* Simple but extensive API analogous to a Command Line Ftp program.
* Supports integration with log4j logging
* EDT Forums are available for public discussion and support. you can free download edtFTPj/Free 2.0.4 now.