Shodan is an Internet of Things search engine that allows you to search and scan a wide variety of devices using a wide array of filters. Some will limit their information gathering to things that they see on the web. You can go beyond this, and Shodan is a tool to help with that: phones, controllers, refrigerators, etc. Shodan has powerful dashboards, community curated filters, and a powerful API to let you plug right into their platform.
Here is a HackerSploit video covering some of the basics of Shodan:
And if you want to check out the engine for yourself...well, here you go!
Link to website: https://www.shodan.io/
This is a nice and quick information gathering tool to perform a reverse email lookup, to see who owns an email address and gather more information about them.
From their site: