Nessus is a free, open-sourced and up-to-date remote security scanner for Linux, BSD, Solaris and some other systems. It is multithreaded, plugin-based, has a nice GTK interface and currently performs over 270 remote security checks.
You can download it from the following link: https://packetstormsecurity.com/files/download/11464/nessus-0.99.1.tar.gz
Source: https://packetstormsecurity.com/files/11464/nessus-0.99.1.tar.gz.html