Slackware is one of the oldest and most mature Gnu/Linux distributions.
Samba is a file and printer sharing protocol that can easily interoperate with Microsoft-based workstations and servers.
Apache is the world's most popular web server.
MySQL is the world's most popular open source database server.
An open source Voice-over-IP Softswitch that can run on Linux, BSD, Mac OS X, and Windows. FreeSWITCH is a very flexible system, and can operate as a switch and as a PBX.
Qmail is an open source mail transfer agent, highly secure, fast, powerful and user-friendly when coupled with vpopmail and qmailadmin.
Symantec is the maker of high quality mail filtering and antiviral protection for Windows-based systems.
Vexira Mail Armor is a snap-in email virus filtering package that works with Qmail.
DSPAM is a self-learning spam filter with Bayesian Noise Reduction capability. It is known to reach accuracy of 99.991%.