If you just need to load classes such that they are available to multiple webapps then put the classes/lib in the "shared" directory (i.e. C:\Program Files\Apache Software Foundation\Tomcat 5.5\shared on a Windows box with the default install).
If you really need a reference to the shared class loader you can get a reference to the webapp class loader (this.getClass().getClassLoader() from a
servlet will do the trick) and then get its parent (getParent()).