Network vulnerability assessment tool
Nessus was one of the most popular free open source vulnerability scanner tool available up until 2008 where it became a commercial tool. Prior to 2008 this tool was used by most professionals when conduction IT Security assessment and still today this tool is classes among the tope tools on the market.
Nessus is still the best UNIX vulnerability scanner available and among the best to run on Windows. Nessus is constantly updated, with oven 20,000 plugging. Key features include remote and local (authenticated) security checks, a client/server architecture with a GTK graphical interface, and an embedded scripting language for writing your own plugging or understanding the existing ones.
This software is available to run both on windows and Linux platforms.
From our experience with this tool we would consider Nessis as a must have tool for running security security assessments.