OpenBSD's ntpd.

The ntpd daemon implements the Simple Network Time Protocol version 4 as
described in RFC 2030 and the Network Time Protocol version 3 as de-
scribed in RFC 1305.  It can synchronize the local clock to one or more
remote NTP servers and act as NTP server itself, redistributing the local
time.

WWW: http://www.openntpd.org/