DTrace

As you may already know, Leopard (a.k.a. Mac OS X 10.5) supports DTrace. DTrace was made by Sun and it is "a comprehensive dynamic tracing framework for the Solaris Operating Environment. DTrace provides a powerful infrastructure to permit administrators, developers, and service personnel to concisely answer arbitrary questions about the behavior of the operating system and user programs."

You can find many information about DTrace here.