OSGi is a standard lightweight API for plugin framework (useful for building microkernels) with a bias toward the needs of network devices.  Recently it gained some momentum when the Eclipse team replaced their original plugin mechanism with OSGi (actually, they paved over it rather than replace).

First full version of Oscar, an implementation of OSGi, was released yesterday.  Also checkout OSGi bundle repository and this nice tutorial of how OSGi can be used.