freesmtp is a decent basic choice. http://www.softstack.com/freesmtp.html

There’s also a sendmail.exe which is a very basic system (not actually sendmail, though it accepts basic sendmail arguments.) I’ve used it for things like sending bugzilla notifications and it works with PHP apps. http://glob.com.au/sendmail/

Mercury mail is a more fully featured smtp server that’s fairly simple to configure (though it takes some hunting through it’s weird menus). It’s the mailserver built by Pegasus, and while not exactly “modern” it’s fairly robust. http://www.pmail.com/whatsnew/m32401.htm

I’ve also used Apache JAMES for Java projects. James itself is fairly lightweight, but the JVM is not. But if you’re already running a servlet container.
http://james.apache.org/

Dumbster is a fake SMTP server (in Java) which dumps email to a file. http://quintanasoft.com/dumbster/

Rumbster is a Ruby equivalent http://github.com/sr/rumbster