Intruder is described as 'security monitoring platform for internet-facing systems' and is an website in the Security & Privacy category. There are more than 25 alternatives to Intruder, not only websites but also apps for a variety of platforms, including Windows, Linux, Mac and SaaS. The best alternative is Burp Suite, which is free. Other great sites and apps similar to Intruder are OWASP Zed Attack Proxy (ZAP) (Free, Open Source), Shodan (Freemium), OpenVAS (Free, Open Source) and Nessus (Paid).
Shodan is a search engine that lets you find specific computers (routers, servers, etc.) using a variety of filters. Some have also described it as a public port scan directory or a search engine of banners.
The Nessus vulnerability scanner is the world-leader in active scanners, featuring high-speed discovery, configuration auditing, asset profiling, sensitive data discovery and vulnerability analysis of your security posture.
Nikto is an Open Source (GPL) web server scanner which performs comprehensive tests against web servers for multiple items, including over 6400 potentially dangerous files/CGIs, checks for outdated versions of over 1000 servers, and version specific problems on over 270 servers.