The best SEO alternative to Nessus is SiteOne Crawler, which is both free and Open Source. If that doesn't work for you, our users have ranked more than 50 alternatives to Nessus, but unfortunately only one of them is a SEO Tool.
Nessus alternatives are mainly SEO Tools, but if you're looking for
Vulnerability Scanners or Website Audit Tools you can filter on that. Other popular filters include
Open Source and
Free. You can also filter by region, for example
EU-based alternatives if you prefer software developed in the European Union. These are just examples - use the filter bar below to find more specific alternatives to Nessus.