Tomcatで共有ライブラリを置く場所。

Apache Tomcat – Apache Tomcat — Migration Guide

Apache Tomcat 5.5.x used, by default, a directory structure including common, shared and server.
Apache Tomcat 6.0.x merges these repositories into a single lib folder.
The conf/catalina.properties file can be used to modify the directory structure.

ねぇと思ったらそういうことかよ。