Optional but recommanded: Apache Commons IO from 1.4 (special file functions like wildcard support), only needed if using Filesystem based default implementation but could be removed by setting the internal configuration as wanted.
optional: DOM4J from 1.6.1 and JAXEN from 1.1.1 (XML support), only used by the current implementation of the standard Ftp Server. It could be replaced very easily.
optional: SLF4J from 1.5.8 and LOGBACK from 0.9.17, can be replaced by any logger facilities (default is Java native logger). Only used by the current implementation of the standard Ftp Server. It could be replaced very easily.