Sun have a ‘CoolStuff’ page on their site dedicated to new and interesting projects that some of their staff are looking at.
I’m not sure how this page is linked to elsewhere (I certainly did not know it existed), but I found a link to it from one of the JavaOne ’04 Sessions that is going to cover one of these Emerging Technology projects, ‘jvmstat’.
J2SE 1.4.1 and above has built in facilities for ‘instrumentation’ of the VM at runtime. The jvmstat project has a selection of interesting tools that allow you to examine the Garbage Collection behaviour of the VM at runtime.