SiteMesh is a lightweight and flexible Java web application framework that applies the Gang of Four decorator pattern to allow a clean separation of content from presentation.
Imagine working with only the simple content of your website and having the complex presentation code applied (decorated) just before delivery to the browser.
SiteMesh uses Filters introduced since Java Servlet version 2.3.
For a good overview, see Mike Cannon-Brookes' PowerPoint presented at the Vegas Server Side Symposium.
Download SiteMesh 2.4.2, a stable, rock solid and Production proven release or check out SiteMesh 3 Alpha.