OpenNTPD aims to be a simple, reliable, and highly secure implementation. It was created as a response to the complexity and age of the code base in the reference ntpd implemenation.
See the OpenNTPD project site for more information.
Limitations
OpenNTPD does not have micro-second accuracy as a design goal, and reference clock support is non-existent.
-- RyanMalayter - 21 Feb 2008