If you are just going for an increase in modularity, I would recommend using a dependency injection container such as PicoContainer, Guice or Spring.
If you want a light-weight plug-in architecture, then go for Java Plugin Framework (JPF).
It allows you to define extension points, which can be implemented by your modules. The primary job of the plug-in framework is to provide a way that you can bundle these modules (as jars), which are dynamically found by the core application and given as implementations of the extension point.
