Proxyma is a multiple reverse-proxy library/webapplication with a cache subsystem and url-rewriting capabilities. The new core is based upon a flexible plugins subsystem easy to extend and customize. Recently this project was promoted to release 1.0.
J2EP is a reverse proxy running on a J2EE engine. The proxy is written in java and was originally designed with Tomcat in mind, but any engine should work fine. A set of basic mapping rules are included but they can easily be extended to your own needs.