Is there a freeware or shareware Web forum application written in Servlet or JSP?

Check out Jive.

From the site:

Jive is forum software that allows threaded discussions on websites. It features excellent performance, user-defined "skins" to completely change the look, feel, and function of forums, and a plugin API that can provide advanced features such as on-the-fly translation, content filters, or almost anything else. Even better, Jive is Open Source. That means that it's completely free to use on your own site, and more importantly, has a dedicated community that continually improves the software.