SpamAssassin
SpamAssassin is a mail filter to identify spam. It is an intelligent e-mail filter which uses a diverse range of tests to identify unsolicited bulk email, more commonly known as Spam. These tests are applied to e-mail headers and content to classify e-mail using advanced statistical methods.  

Spamato
Spamato is a spam filter system that combines several anti-spam techniques, from which thousands of users benefit. It is available as an add-on for Thunderbird and Microsoft Outlook, and as a stand-alone proxy for other e-mail clients. IMAP and POP are supported.

Webmin
Webmin is a web-based interface for system administration for UNIX. Using any modern web browser, you can setup user accounts, Apache, DNS, file sharing and much more.

SARG
Squid Analysis Report Generator is a tool that allows you to view "where" your users are going to on the Internet. SARG generate reports in html showing users, IP addresses, bytes, sites and times.