Greasemonkey

Greasemonkey is a Mozilla Firefox extension that allows users to install their own Javascripts to websites.

You can find user scripts on userscripts.org, and LiveJournal oriented scripts as well.

People interested in writing their own Greasemonkey scripts are encouraged to read Dive Into Greasemonkey.

There are Greasemonkey like extensions or capabilities for other browsers, including Creammonkey for Safari, User Javascript for Opera, and GreasemonkIE, Trixie, and Turnabout for Internet Explorer.

Sample Greasemonkey Script: // Begin Userscript Comments // ==UserScript== // @name         Script Name // @namespace    http://www.example.com/    // URL of Script Author // @description  Description of Script // @include      http://www.google.com/*    // URLs to include // @exclude      http://images.google.com/* // URLs to exclude // ==/UserScript== // End Userscript Comments // Begin Userscript Functions (function {  // Begin Helper Functions   function my_decodeURI(uri) {     return decodeURI(decodeURI(uri));   }   // End Helper Functions   // Begin Main Script   var as = document.getElementsByTagName('a');   for(var i = 0; i < as.length; i++) {     var a = as[i];     a.href = my_decodeURI(a.href);   }   // End Main Script }); // End Userscript Functions