Shodan is the world's first search engine for crawling, IP lookups, searching, data streaming.
2. Portswigger
Free, online web security training from the creators of Burp Suite.
3. Have I Been Pwned?
A website that allows to check email ID's or data has been compromised in data breaches.
4. OWASP
The Open Web Application Security Project® (OWASP) is a nonprofit foundation that works to improve the security of software.
Through community-led open-source software projects, hundreds of local chapters worldwide, tens of thousands of members, and leading educational and training conferences, the OWASP Foundation is the source for developers and technologists to secure the web. - Tools and Resources - Community and - Networking Education & Training
5. VirusTotal
VirusTotal search items with over 70 antivirus scanners and URL/domain blocklisting services. VirusTotal is free to end users for non-commercial use.