Zenoss is an enterprise network/systems management application that provides an integrated software package for monitoring availability, performance, events, and configuration across layers (network, servers, applications, environment, etc.) and across platforms (Linux, Windows, Unix, etc.).
GroundWork Monitor Community Edition can give you insight into your computing infrastructure, allowing you to see the current and historical states of all your computers: servers, desktops, and laptops, all of your network devices, all of your services (like TCP/IP and Web services), and all of your applications (like mail servers and database apps). You can choose to be alerted when something goes awry via pager, SMS, email, or phone, and even set up automatic restarts or fall-overs.
Cyan Secure Web Proxy Server is a carrier grade, high performance Internet filtering proxy server for Linux. It includes scalable (user/group/host) Web filter and virus scan utilities for blocking malicious applications at the gateway. It has an advanced URL database, authentication support (Active Directory, LDAP, NTLM), SSL Interception, easy deployment, and remote administration.