OpenSMTPD part of the OpenBSD Project.
Postfix attempts to be fast, easy to administer, and secure. The outside has a definite Sendmail-ish flavor, but the inside is completely different. Built from source code, Postfix can run on UNIX-like systems including AIX, BSD, HP-UX, Linux, MacOS X, Solaris, and more.
Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet. It is freely available under the terms of the GNU General Public Licence.