Integrated into Sendmail
With Sendmail, Spamassassin can be called for every user by editing /etc/procmailrc . More details at http://www.stearns.org/doc/spamassassin-setup.current.html#sitewide
An alternative is to use the smtp-vilter high-performance content filter using the sendmail milter API which has a backend for Spamassassins spamd. More details at http://www.etc.msys.ch/software/smtp-vilter/
Integrated into Postfix
Integrated into Qmail
Can be integrated via qmail-scanner, an add-on to qmail. Info available at http://qmail-scanner.sourceforge.net/