Nessus is described as 'Identify system vulnerabilities with automated assessments and prioritize remediation through a scoring system, offering guidance for all experience levels' and is a vulnerability scanner in the network & admin category. There are more than 50 alternatives to Nessus for a variety of platforms, including Web-based, Linux, Windows, SaaS and Mac apps. The best Nessus alternative is SiteOne Crawler, which is both free and Open Source. Other great apps like Nessus are Metasploit, OpenVAS, PhoneSploit Pro and Whatoblock.