Cross domain iFrames
Gadget to Gadget This document explains how to use the publish-subscribe ("pubsub") framework to pass messages from one gadget to another. This feature is deprecated and is unsupported. The publish-subscribe framework allows publisher gadgets on iGoogle to communicate changes to subscriber gadgets that have declared interest in those changes. Note: You can use the publish-subscribe framework for type="html" gadgets.
YouTube iframe API
An update in the era of HTML5 (May 6, 2011) This post has been heavily commented and linked to over the years, and continues to receive a ton of traffic, so I should make it clear that much of this is no longer relevant for modern browsers. On the one hand, they have adjusted and tightened up their security policies, making some of the techniques here no longer relevant. On the other hand, they have introduced technologies that make it easier to do cross-domain communication in the first place. With modern browsers, you can and should be using postMessage for this purpose. Library support is now available too. IFrames - Cross-Domain Communication
Cross-Browser Comm Demo via 'Marathon' technique