SiteMesh is a lightweight and flexible web application framework that applies the Gang of Four decorator pattern to allow a clean separation of content from presentation.
SiteMesh is easy to learn. 5 minutes to setup, and 10 minutes to start being useful.
SiteMesh may be simple to start using right away, but there is much more to SiteMesh...
For a good top level view of SiteMesh, review Mike Cannon-Brookes PowerPoint presentation presented for the Server Side Symposium.