qpsmtpd is a qmail-smtpd replacement written in perl. The new features over the normal qmail-smtpd are primarily tools to avoid accepting mails we have to bounce anyway and a object oriented system for easily writing extension plugins.
I maintain an (incomplete) list of plugins written by various people.
There is also a short Tutorial on setting up qpsmtpd with xinetd and postfix in German.
qpsmtpd produces humongous log files, which contain much more information than those written by qmail or sendmail, so I wrote some tools to analyze log files.