MX ToolBox: All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
PhishTank: Phishing Website Checker
SSL Labs: Test the TLS/SSL Security of your website.
Url Tools: A free service to scan and analyse websites
Url Void: Website Reputation Check
Talos: Cisco Talos Reputation Checker
Securi: Website security and malware scanner
DNS Check: Check DNS Propagation throughout the Globe. Very useful if you updated hosting or DNS records.
VirusTotal: Online Malware Scanner
Hybrid Analysis: Online Malware Scanner
Joe Sandbox: Online Malware Scanner and SandBox
Any.Run: Interactive Malware scanner. Registration Required
Cookoo: Local Automated Malware Analysis. You can throw any suspicious file at it and in a matter of minutes Cuckoo will provide a detailed report outlining the behavior of the file when executed inside a realistic but isolated environment.