DUE TO GRAPL.IO GOING OFFLINE, THIS MOD NO LONGER FUNCTIONS AS INTENDED. ACTUALLY, IT DOES NOTHING AT ALL.
This mod is a utilities mod. Basically, it adds one command, /grappl <open|close> . When on a Single player world, this command will allow anyone to join as long as they have the ip printed in chat.
This mod makes heavy use of a project by Cactos, called grappl.io. Thanks to him, people can now host servers without port forwarding. To download grappl and use it outside mc, or to see the project visit: http://grappl.io/