Cross domain iFrames
Get flash to fully experience Pearltrees
This document explains how to use the publish-subscribe ("pubsub") framework to pass messages from one gadget to another.
Note: The Google Mapplets API has been officially deprecated as of May 19, 2010.
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.