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 Metasploit. It's not free, so if you're looking for a free alternative, you could try Metasploit or SiteOne Crawler. Other great apps like Nessus are PhoneSploit Pro, ScanAnchor, Whatoblock and Sucuri.