Really, I'm not sure something like this makes sense. It's like saying, what is the practical online demo for object orientation?
Modularity is a design approach. It focuses on cohesion and coupling. OSGi technology doesn't provide you with anything cool out of the box, it only provides you with a toolbox to create enforced, modular (and potentially dynamic) systems. You should think of it as more of a layer above the JVM. You don't use OSGi for the sake of using OSGi, no more than you use Java for the sake of using Java.