Nessus is a vulnerability scanner written by Renaud Deraison, founder of Tenable Network Security. It was originally released as an open source project. However the license changed to a closed source one. The product continues to be free for personal usage. However the commercial version runs $2190 a year.

Nessus is available on Windows and Linux based systems. It is purportedly the most popular vuln scanner. It can scan for a number of things such as remove control and access, misconfiguration, bad passwords, and Denial of Service exploits. The tool can help you prepare for a security audit.

You write tests for Nessus to perform. The tests are written in a custom scripting language called NASL. You can subscript to receive tests from Tenable. The program output can be formatted in text, HTML, or even XML formats. A free competitor in the vuln scanning space is OpenVAS.