When building server side applications for the Web, independently of the language used (Java, PHP, Perl, etc), a lot of time is spent writing embedded HTML code and dealing with the fact that HTTP is a stateless protocol. SPFC allows you to write HTML forms like you write a Java GUI.
Apache Cocoon is a Web development framework built around the concepts of separation of concerns and component-based Web development. Cocoon implements these concepts around the notion of "component pipelines", each component on the pipeline specializing on a particular operation. This makes it possible to use a Lego(tm)-like approach in building Web solutions, hooking together components into pipelines without any required programming.
CustomDNS is a modular DNS server written in Java. It's based on Brian Wellington's dnsjava pacakge, but it adds support for virtual hostnames, SQL databases, and dynamic client updates. You can use CustomDNS out-of-the-box as a clone of ml.org/dyndns.org, or you can use it to develop custom DNS applications.