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.
Chev is a sane by default vulnerability check script that checks NIST and US Cert RSS feeds for a short list of software that you configure. It has two levels of priority, warn and critical, which allows for easy use with Nagios as a stand-alone script or in a cron job. Just add strings for your major pieces of software and chev will tell you when a vulnerability has been released.