The OSGi framework mentioned is very cool indeed. It's best known usage is the Eclipse
IDE. It can also be used in web applications, where especially the Wicket
component web framework delivers a very good integration. There are several users working with OSGi compliant frameworks (most notably Oscar, which is in the Apache incubator
under the name Felix), and Wicket. I have used Oscar and Wicket in a commercial product and we were very satisfied with the runtime re-deployment of new components.