DTrace in FreeBSD 7.1-RELEASE
From the release notes of FreeBSD 7.1-RELEASE:
>The DTrace, a comprehensive dynamic tracing framework and dtrace(1) userland utility have been imported from OpenSolaris. 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.
(via Tim Bray)