Search engines
Shodan
Shodan is a search engine for Internet connected devices such as servers, routers, webcams, IoT devices.
Censys
Internet-connected hosts, websites, certificates, and other Internet assets. Some of its use cases include enumerating domains in use, auditing open ports and services, and discovering rogue assets within a network.
Virus Total
Provides a virus-scanning service for files using multiple antivirus engines. It allows users to upload files or provide URLs to scan them against numerous antivirus engines and website scanners in a single operation. They can even input file hashes to check the results of previously uploaded files.
Have I been pwned
Tells you if an email address has appeared in a leaked data breach.