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 approximate IP details.
Get all possible details about a domain name.
Ping a website, server or port..
Get the gravatar.com globally recognized avatar for any email.
Check if the URL is cached or not by Google.
Get all possible details about an SSL certificate.
Get & verify the meta tags of any website.
Make sure your passwords are good enough.