Ubiquity, a free work-in-progress from Mozilla Labs, adds the ability to type in natural-language commands for things like mapping an address or translating text to Firefox.
Ubiquity's utility doesn't lie in performing any particular amazing task. It won't do anything for you that you can't already do by bringing up a Web page, winding through Firefox's command menus, or using one or more existing hotkeys. Instead, it makes doing things you already do easier and faster.
After you install the Ubiquity add-on into Firefox, hitting Ctrl-space brings up an overlay where you can type commands. The commands are based on normal language, such as "map [address]" to bring up a Google map of a particular address, or "translate (word) from english to french." The overlay will help guide you to available commands with user-friendly features such as listing potential commands as you type, and a first-time use tutorial will help you learn the ropes (one suggestion: maximize your browser before running the tutorial).
Ubiquity comes with plenty of built-in commands, and also allows for adding new commands created by other Ubiquity users. But you'll probably want to be at least slightly comfortable with Web development and programming before trying to create your own commands.
The promising Ubiquity tool is based on the excellent Enso Launcher, which uses a similar approach for opening programs, checking spelling or performing other tasks in Windows.