Relatively painless magnet uri generation for torrentz.eu. There's some extra scaffolding for extension related stuff but this is the main guts.

rewrite.js

// Relatively painless magnet uri generation for torrentz.eu. Digging reveals
// that the torrent hash and id (torrentz.eu specific) are stored as hidden
// inputs for the comment system. All that's required is some Ajax and string
// parsing for the announce list and some html injection to add the link to
// the page.

// Get the torrent name, hash and id (for announce list lookup)
var name = $(".download>h2>span").first().text();
var hash = $(':input[name="hash"]').first().val()
var torrent = $(':input[name="torrent"]').first().val()

$.ajax({
	url: "https://torrentz.eu/announcelist_" + torrent,
	datatype: "text"
}).done(function(announcelist) {
	// Parse the tracker list
	var trackers = $.trim(announcelist).split("\n\n");

	// Create the magnet uri. 'urn:btih' gets url encoded when it shouldn't so
	// just create this parameter manually.
	var magnet = "magnet:?xt=urn:btih:" + hash;
	magnet += "&" + $.param({
		dn: name,
		tr: trackers
	}, true);

	// Add new source entry to list
	$("div.download>dl:first").after(
		$('<dl/>').append(
			$("<dt/>").append(
				$("<a/>", {
					href: magnet,
					rel: "e",
					target: "_blank"
				}).append(
					$("<span/>", {
						'class': "u",
						style: "background: transparent url('" + chrome.extension.getURL("icon16.png") + "') no-repeat 5px center;",
						text: "Magnet URI"
					}),
					$("<span/>", {'class': "n", text: name})
				)
			)
		)
	);
});