While building Mashup or building Portals this may be a common development problem. There are 2 ways to solve this 1) Through JSON 2) Through iframe hack - http://www.julienlecomte.net/blog/2007/11/31/ – This uses YUI

JQuery plugin for the solution number 2 - http://code.google.com/p/jquery-crossframe/

HTML5 supports this through postMessage - see Ajaxian post here - http://ajaxian.com/archives/cross-window-messaging-with-html-5-postmessage