First you need API key from http://openweathermap.org
of course..you do not have to use openweathermap..It was just an example
you can use Weather Underground,Yahoo! Weather...or
4 Free Weather Providers API To Develop Weather Apps
apparently it's possible to recive XML with Ajax from openweathermap...
so there is no need for additional JSON plugin..
(tested in Chrome,FireFox,&Opera)