Very simple disk synchronization suitable for maintaining a backup directory. I built it to keep Photos and MP3s on my NAS in sync with my laptop and desktop. Will do one-way or two-way (mirrored), but does not delete. All Java, no GUI.
Java Virtual System (JVS) is built around a virtual file system (VFS) - a common platform of Java, by Java, for Java. JVS extends JVM and makes it from an interpreter running half-compiled code a first-class server.
Mudslide provides an encrypted file system to any operating system that can mount a webdav server. Mudslide works with Linux, OS X, and Windows. Mudslide is built upon http://jakarta.apache.org/slide/ and the encryption libraries packaged with Java.