ClassGhost is a small utility that can redefine class files within a running JVM. ClassGhost allows you to directly inject the newly compiled class files in to the VM without interrupting the running application. Works with multiple as well as remote VMs
BeanClipse is a NetBeans plugin for Eclipse: NetBeans runs embedded within the Eclipse platform as a plugin offering the best of both worlds to Java developers.
May be just a joke ;-) ....
Check also it's twin at : http://sf.net/projects/netclipse
xsldb is a fully featured xslt debugger that can easily be linked into every xslt-development environment that uses Apache XALAN or the Sun JDK implementations. xsldb is interactive, console based and offers an intuitive, easy to learn debugging language