The Nessus security scanner is a free, open-sourced and easy-to-use security auditing tool for Linux, BSD and some other systems. It is multithreaded, plugin based, has a nice X11 interface and performs over 250 remote security checks.
You can download it from the following link: https://packetstormsecurity.com/files/download/11585/nessus-0.98.4.tar.gz
Source: https://packetstormsecurity.com/files/11585/nessus-0.98.4.tar.gz.html