Google is a search engine for human readable content. Shodan does the same for machine readable content.
You can:
- search for specific IP addresses, and it will show you the active ports, and running services.
- search for a specific response header in the set of countries
- browse through the screenshots of open VNC & RDP
- open webcams
- and more...
The free accounts can use any feature, but the list of results is limited. But if you really want to look under the deck of the internet, the subscription is worth it.
One random use I found years ago was that I could instantly search for Comcast business boxes and then export the IPs as a csv, run a headless browser script to try default login creds, then scrape the wireless Mac, ssid, and password which I could plot on a map using wigle wifi wardriving data. It's pretty cool. Maybe the monitoring service will come in handy if it ever notifies me of anything I should be concerned about on any of my IPs, hard to turn down a $5 lifetime pass.