Starting with Java 5, it is possible to add Java agents to the class loader.
Have you written any Agents? have you used any agents? What is the interesting use of Agents?
I used various java profilers that use this function. A good open source Java open source profile that uses this is Profiler4j ( http://profiler4j.sourceforge.net ).
This is a great tool for finding performance bottlenecks in your Java code.