A prototype of the Ubiquity extension for Firefox was released today. This is still version 0.1 so things do not work perfectly. The potential for this is enormous and the Internet has been buzzing about it all day.
It is my understanding that Ubiquity basically allows you to mashup various components of the web via a command line interface that is integrated with the browser. As the video outlines, if you want to make a lunch appointment with your friend via Gmail (it does not currently work with other email clients) you can insert a map to the location, a review of the restaurant, and add the appointment to your google calendar from within the body of the email using the extension. Prior to this, you would need to add links to these various objects by opening news tabs and cutting and pasting the information.
Please watch the video for a better explanation of how it works.