Here is an incomplete listing of the external dependencies for Jetspeed 2 and the reason for their use.
pluto-1.0.1-SNAPSHOT.jar: Jetspeed 2 implements the Pluto Portlet Container to interact with JSR-168 portlets.
castor-0.9.4.3.jar: Is used to marshal and unmarshal .psml pages for portlet aggreagation.
commons-digester-1.5: Is used for the reading reading of these deployment descriptors: portlet.xml, web.xml and jetspeed-portlet.xml
commons-configuration: Used to read the jetspeed.properties configuration file.
commons-collections: Required by OJB.
commons-httpclient: Used by the org.apache.jetspeed.pamanager.servletcontainer.TomcatManager to manage web apps within tomcat and org.apache.jetspeed.util.velocity.RemoteContent class.
commons-logging-1.0.3: Primary logging mechanism in Jetspeed 2.