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. IFrames - Cross-Domain Communication
Cross-Browser Comm Demo via 'Marathon' technique