Minify your HTML by removing all the unnecessary characters.
Minify your JS by removing all the unnecessary characters.
Easily convert IDN to Punnycode and back.
Extract email addresses from any kind of text content.
Find A, AAAA, CNAME, MX, NS, TXT, SOA DNS records of a host.
Make sure your passwords are good enough.
Take an IP and try to look for the domain/host associated with it.
Get all possible details about an SSL certificate.