Default packet filter (aka firewall with NAT) from OpenBSD 3.0 (December 2001)
Replaced IPFilter, which had to be removed due to licensing issues (which in turn lead to a license audit of the entire OpenBSD source tree)
Based on new code by Daniel Hartmeier (June 2001 ->), since hacked on by several others
High performance (see http://www.benzedrine.cx/pf-paper.html), low maintenance