OpenBSDs web http://www.openbsd.org/
OpenBSDs FAQ, http://www.openbsd.org/faq/index.html
PF User Guide http://www.openbsd.org/faq/pf/index.html
Daniel Hartmeier's PF pages, http://www.benzedrine.cx/pf.html
Daniel Hartmeier: Design and Performance of the OpenBSD Stateful Packet Filter (pf), http://www.benzedrine.cx/pf-paper.html (presented at Usenix 2002)
Nate Underwood: HOWTO: Transparent Packet Filtering with OpenBSD, http://ezine.daemonnews.org/200207/transpfobsd.html
Randal L. Schwartz: Monitoring Net Traffic with OpenBSD's Packet Filter, http://www.samag.com/documents/s=9053/sam0403j/0403j.htm
Unix.se: Brandvägg med OpenBSD, http://unix.se/Brandv%E4gg_med_OpenBSD
Randal L. Schwartz: Blog for Thu, Jan 29, 2004, http://use.perl.org/~merlyn/journal/17094
RFC 1631, "The IP Network Address Translator (NAT)", May 1994 http://www.ietf.org/rfc/rfc1631.txt?number=1631
RFC 1918, "Address Allocation for Private Internets", February 1996 http://www.ietf.org/rfc/rfc1918.txt?number=1918
The FreeBSD PF home page, http://pf4freebsd.love2party.net/
Peter Postma's PF on NetBSD pages, http://nedbsd.nl/~ppostma/pf/
Marcus Ranum: The Six Dumbest Ideas in Computer Security, September 1, 2005
Kjell Jørgen Hole WiFi courseware, http://www.kjhole.com/Standards/WiFi/WiFiDownloads.html
Greylisting.org greylisting.org is the home of all things greylisting, with links to numerous articles and other useful information.
Evan Harris: The Next Step in the Spam Control War: Greylisting (the original greylisting paper)
Mark Uemura: What's New in 4.3: authpf-noip
Henning Brauer: Carp and STP meet switch security
Michael W. Lucas: Network Flow Analysis, No Starch Press 2010
Peter N. M. Hansteen: Effective Spam and Malware Countermeasures - Network Noise Reduction Using Free Tools - originally a paper presented at BSDCan 2007 which puts spamd into a slightly wider spam and malware fighting context along with some data on spammer behavior, this 2013 update contains some updates relevant to present conditions.
Peter N. M. Hansteen: The Book of PF, No Starch Press 2014 (second edition in November 2010, first published 2007), is an expanded and extensively rewritten followup to the tutorial, and covers a range of advanced topics in addition to those covered here.