In Firefox it is possible to add extra functionality to web applications with extensions such as [Greasemonkey](https://addons.mozilla.org/en-US/firefox/addon/748/) and [Stylish](https://addons.mozilla.org/en-US/firefox/addon/2108/). [Opera](http://www.opera.com/docs/userjs/) also has this feature natively. Which of these types of scripts do you find the most useful?