HackerOne - HackerOne provides a platform designed to streamline vulnerability coordination and bug bounty program by enlisting hackers.
Whoisology - Connected database of whois records and whois history
Acunetix - Audit your website security and web applications for SQL injection, Cross site scripting and other...
WhoisThisDomain - Domain name lookup tool that allows you to get information about domain registration from WHOIS server.
Trustwave Services - Trustwave is a leading cybersecurity and managed security services provider that helps businesses fight cybercrime, protect data and reduce security risk.
JSON WHOIS API - Parsed domain WHOIS data from any WHOIS server via RESTful API with metered pay as you pricing.