And with DTrace too!

And now I notice that Leopard has DTrace too!

To help you understand how the system behaves while your application runs, Leopard comes with DTrace, a comprehensive command-line monitoring utility that can probe almost any aspect of system performance. DTrace includes a large set of tools that cover most general aspects of system performance, including kernel and user space code. If there is an aspect of the system that you want to track that isn't already supported, you can create your own tool using the D programming language.

Note that the built-in Ruby comes with DTrace probes. This means you can trace the execution of your app from Rails, all the way down the Kernel and back.