Minify your HTML by removing all the unnecessary characters.
Minify your JS by removing all the unnecessary characters.
Find A, AAAA, CNAME, MX, NS, TXT, SOA DNS records of a host.
Ping a website, server or port..
Get approximate IP details.
Get all possible details about a domain name.
Take an IP and try to look for the domain/host associated with it.
Check if the URL is cached or not by Google.