Sometimes you are behind some paywall/captive portal/firewall and you feel like certain pors would be left through if only you knew which of the over 65,000 ports did. The only way to really know is to check each one individually. Thats where http://portquiz.net/ comes in.
It's a site that registers every port as open. This way you know that if something is allowed through, it will come back in your port scan.
So behind your firewall, this:
now you can check the firewallcheck.nmap (or parse it out of gnmap) and find out which ports allow data through.