Greasemonkey + jQuery
It used to be difficult to combine Greasemonkey and jQuery (or any other javascript library). Looking into it again, I came across 3 methods and this comment from Stephan Sokolow:
For the record, the newest Greasemonkey versions should now provide a much better alternative to this.
See http://wiki.greasespot.net/Metadata_block#.40require for an example of how to efficiently load jQuery from the Google AJAX API … at userscript install time via the @require metadata key.
It really is very simple now—an example from the page linked aboved:
// ==UserScript==
// @name Hello jQuery
// @namespace http://wiki.greasepot.net/examples
// @description jQuery test script
// @include *
// @require http://ajax.googleapis.com/ajax/libs/jquery/1.2.6/jquery.js
// ==/UserScript==
$(document).ready(function() {
$("a").click(function() {
alert("Hello world!");
});
});
And here’s a script I wrote which takes the top 100 movie torrents from The Pirate Bay and adjusts the size of each link based on its IMDB rating:

Install the Greasemonkey script and visit The Pirate Bay’s Top 100 Movies page to see it in action. You can also view the source.