- greasemonkey

"Greasemonkey is a Firefox extension which lets you to add bits of DHTML ('user scripts') to any webpage to change it's behavior. In much the same way that user CSS lets you take control of a webpage's style, user scripts let you easily control any aspect of a webpage's design or interaction."

Well, that's extremely interesting. Ran across it because BlogMarks is offering a replacement for Bloglines' Clip/Blog This function. That is (for users who opt-in) they're replacing a piece of functionality in another web app with their own. Imagine if there was a widespread facility to do that.