Take an IP and try to look for the domain/host associated with it.
Find A, AAAA, CNAME, MX, NS, TXT, SOA DNS records of a host.
Get all possible details about an SSL certificate.
Get all possible details about a domain name.
Ping a website, server or port..
Check if the URL is cached or not by Google.
Get details of any file type, such as the mime type or last edit date.
Make sure your passwords are good enough.
Get the web-host of a given website.
Check if the URL is banned and marked as safe/unsafe by Google.
Parse details from user agent strings.