JBoss was possibly the first to introduce a non-monolithic J2EE implementation.  The implementation was developed on top of JMX based infrastructure and dubbed a Microkernel architecture.  The clear advantage of such an approach was to allow best of bread solutions to be plugged in.  The JMX based microkernel design for J2EE was later copied by HP and Macromedia.

The JMX microkernel approach was also leveraged by Apache's Avalon project to build more generic servers (i.e. email, directory, web, jabber etc.).  The Compiere project a ERP-CRM application also chose to implement its services in terms of JMX managed services.

The Eclipse IDE also chose a microkernel design to manage the various plugins that can be dynamically added to its workbench.  Realizing the potential of such a design, Eclipse has embarked in a project called Equinox to explore this even further.

Howard Ship the developer of Tapestry, a Web Application framework who's claim to fame is its highly componentized nature, is embarking in a new project HiveMind.  He's studied the JBoss and Eclipse approaches and believes he can do even better!Meanwhile, Sapient the Web Consulting company has relased its own open source version of a microkernel framework named Carbon. Another group, spinning off from JCorporate, is introducing a "meta-framework" named Keel leveraging the Avalon microkernel.  In short we are seeing the emergence and thriving of a new species of software construct.

Also, in an earlier piece about Aspects vs. Components, I forgot to mention that an Aspect based approach to building Containers is analagous to building a modular as opposed to a monolithic Container.

This leads me to another fearless forecast.  The combination of Aspect Oriented Programming and Microkernel Architectures will lead to the extinction of the current monolithic J2EE containers that we've known and loved (or despised?) so well. 

If you didn't quite grok what I just said, then try visualizing the passing away of dim-witted lumbering dinosaurs with adaptive, nimble mammals emerging in the background.  Hope you get the picture.