Greg Miller at MacTech - Exploring Leopard with DTrace:

"A few years back, Sun Microsystems developed DTrace: a new and innovative way to trace running software on live systems. DTrace enables developers and administrators to "see" what their code, and others' code, is doing in a flexible and dynamic way. With the release of Leopard, Apple has brought DTrace to Mac OS X.

This article will begin with a crash course in DTrace. If you're already a seasoned DTrace veteran, feel free to skip that section. We will then move on to some examples of how to use DTrace by exploring our Leopard systems and discovering what makes them purr."