Definetely handy when you want a sendmail-compatible (ie /usr/sbin/sendmail) MTA for a firewall type machine that should only be send-only, no daemon (although this supports receiving as well). An example is a firewall machine w/Arpwatch -- it requires /usr/sbin/sendmail to exist, but who wants to install all of sendmail just to send a status message? Well done.
Smail-3 definitely came after Sendmail.
It was written as a sendmail replacement for normal people and as such has a much simpler configuration interface insted of the finite state machine that drives sendmail. in fact the most recent versions should work out of the box with no post-compilation configuration necessary on most leaf node sites.
The major version number of Smail is "3" because it came after Smail-2, a very simple UUCP mailer written a very long time ago by Chris Seiwald when he was at AT&T in order to do automatic UUCP routing using pathalias and the UUCP Map Project database. Smail-2 replaced an earlier Smail-1. Smail-1 was probably written just about the same time, or maybe before, sendmail, but I'm not sure (sendmail is first copyright in 1983 and smail-2 is first dated 1985). Smail-3's authors liked the simplicity of Smail-2 and the idea was to write a mailer to replace Smail-2 and bring new gateway and Internet capability without sacrificing the simplicity. Other than that Smail-2 and Smail-3 are really linked only in name.
Most other currently used SMTP-capable mailers, including zmailer, exim, qmail, postfix, and so on were written long after sendmail was first released and most were written to address the shortcomings of sendmail. Exim was in fact modeled after Smail-3 and could in some senses be considered to be a full rewrite.
An open, cross-platform journaling program.
A scientific plotting package.