Part I of this article “SPF Overview” covers publishing SPF on your name server: linuxjournal.com/article/7327
libspf: libspf.org
Milter: www.milter.org
SPF Download Page: spf.pobox.com/downloads.html
SPF wizard: spf.pobox.com/wizard.html
SRS: spf.pobox.com/srs.html