The Mozilla Labs is experimenting with a new Firefox extension called Ubiquity (http://labs.mozilla.com/2008/08/introducing-ubiquity/). This extension allows you to open a command window in the browser and perform very interesting mash-ups.
The test version, see the video, brings features such as quickly searching Wikipedia, searching addresses on Google Maps and others. The best part is that users can define new commands, see the tutorial (https://wiki.mozilla.org/Labs/Ubiquity/Ubiquity_0.1_Author_Tutorial).
If you want to insert maps into your Gmail, translate passages from web pages, search for books on Amazon without leaving the current page, Ubiquity may have been made for you.