In AdGrok’s GrokBar, we inject a “heads-up display” on pages that the user is advertising. The heads-up display is actually an iframe that’s positioned within a browser-extension-injected div, and that iframe renders content from our secure server farm. You can see a demo video here (and see that we truly spared no expense on the voice-over talent!).
I wanted to make our extension’s button-click incant a javascript method that was inside the GrokBar’s iframe. I found this horrible hack, but every time a javascript timer scrapes a hidden DOM element, or mucks with the URL fragment of an iframe src in order to send messages, the code gods kick a puppy.