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 25 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 Sucuri.
Better features, open API.