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.
Check if the URL is cached or not by Google.
Get approximate IP details.
Take an IP and try to look for the domain/host associated with it.
Get all possible details about an SSL certificate.
Get all the HTTP headers that an URL returns for a typical GET request.