Yes, EnterpriseDT's edtFTPj is stable (first released in 2000), has all the features you might need, and is open source as well.
It's used in a bunch of open source projects (as well as in many commercial projects), and is acknowledged to be one of the fastest client libraries around.
As another poster noted, if you do wish to upgrade to SFTP and/or FTPS, it is a simple upgrade path with very few code changes required.