Ubiquity is an experimental Firefox extension that gives you a powerful new way to interact with the Web.
You're used to telling Firefox where you want to go by typing Web addresses into the URL bar:
With Ubiquity installed, you'll be able to tell Firefox what you want it to do by typing commands into a new Ubiquity input box.
The commands are pretty straightforward, but you can define your own and then chain them together and do things that are pretty mindsnappingly powerful.
Having a browser that "understands" natural language has enormous potential. Ubiquity is in the earliest of prototype stages, but seems stable and is already a useful tool in my online kit.