There's plenty of advocacy for Postfix and Dovecot on this list; I occasionally like to chime in with a bit for Courier MTA: http://www.courier-mta.org/
Among the advantages I appreciate: * Maildrop is much easier to manage than procmail * Configuration is much simpler: it's substantially similar to the highly regarded Qmail * The entire system is a single integrated package, so you only have to configure things like authentication once rather than for each server (as in postfix and dovecot). * Courier supports an SMTP filtering API that's much simpler than either Postfix or Sendmail. I wrote courier-pythonfilter to help email admins filter and modify messages using Python. ;)