DTrace is a tracing system implemented in the Solaris kernel that allows tracing of user level and kernel level programs. It is dynamic, meaning that it is explicitly called and when not in use it consumes no extra resources.
The architecture is two tiered, with a core DTrace module (that acts as a multiplexer and disables interrupts) and providers, which are the modules that actually perform the tracing. In this way, different instrumentation methodologies can be added. Many different providers were already implemented by the authors.
Users specify arbitrary predicates to monitor their programs using a C-like programming language called "D".
This architecture is interesting. I think one recurrent tradeoff in the tracing systems is the amount of user work versus the overhead of tracing.