There's a little bit of buzz going on about Ubiquity, a UI enhancement for Firefox. Firefox already has hundreds of UI enhancements, but this one is a little different. If you've used Quicksilver on Mac OS X, you may notice Ubiquity feels somewhat similar to it. Basically, you can take something (a piece of a web page) and quickly perform an command with/on it.
The commands are nothing more than javascript snippets. But, because usable web APIs are growing in number and usefulness, Ubiquity has the opportunity to create and fill a need we never knew we had. There are already Firefox extensions that do a lot of what Ubiquity can do now, but Ubiquity is a framework, like greasemonkey, that allows developers to quickly build usability tools for users.
For a 0.1 release, Ubiquity comes with lots of commands. Working with search engines, social web ...