Take an IP and try to look for the domain/host associated with it.
Get approximate IP details.
Get all possible details about an SSL certificate.
Get all possible details about a domain name.
Ping a website, server or port.
Find A, AAAA, CNAME, MX, NS, TXT, SOA DNS records of a host.
Make sure your passwords are good enough.
Get & verify the meta tags of any website.
Get the gravatar.com globally recognized avatar for any email.
Get details of any file type, such as the mime type or last edit date.