Apple released Java VisualVM as part of Java for Mac OS X 10.5 Update 4. To launch Java VisualVM, run
jvisualvm from the command line. This Java update is available via Software Update and manual download from http://support.apple.com/downloads/Java_for_Mac_OS_X_10_5_Update_4.
There is Getting Started with VisualVM for those those who want to get more information about the tool.