MultiJava is an open project to add open classes and symmetric multiple dispatch to Java.

Open classes allow one to add to the set of methods that an existing class supports without creating distinct subclasses or editing existing code.  Multiple dispatch, found in languages such as allows the method invoked by a message send to depend on the run-time classes of any subset of the argument objects.

In concert with AspectJ this would appear to add some very powerful capabilities to Java.

Of course Greenspun might argue that you should just use Lisp in the first place... ;-)