OpenSMTPD is a FREE implementation of the server-side SMTP protocol as defined by RFC 5321, with some additional standard extensions. It allows ordinary machines to exchange emails with other systems speaking the SMTP protocol.
Started out of dissatisfaction with other implementations, OpenSMTPD is a fairly complete SMTP implementation.
OpenSMTPD is primarily developed by Gilles Chehade and Eric Faurot, with contributions from various OpenBSD hackers and members from other communities.
OpenSMTPD is part of the OpenBSD Project. The software is freely usable and re-usable by everyone under an ISC license.
A framework used by penetration testers for building custom exploits for infiltrating systems. Written in Ruby. Comes with a large library of payloads and other nifty and fascinating tools. It's worth learning to use if you're serious about penetration testing or exploit development. Also, the cutting edge of attack technologies winds up coming out of the Metasploit project.