Userscript that enables sensible defaults for jsFiddle.

jsfiddle-defaults.user.js

// ==UserScript==
// @name Sensible defaults for jsFiddle.
// @author Mathias Bynens <http://mathiasbynens.be/>
// @link http://mths.be/bde
// @match http://jsfiddle.net/*
// ==/UserScript==

// Ignore existing fiddles
if (window.location.pathname == '/') {

	// Insert JS before </body> and don’t use any libraries by default
	[].forEach.call(document.querySelectorAll('#js_wrap option[value="b"], #js_lib option[value="11"]'), function(el) {
		el.selected = true;
	});

	// Don’t use the “normalized” CSS (it’s more like a reset anyway)
	document.getElementById('id_normalize_css').checked = false;

}

I was asked how to disable the sidebar by default. You could add this to the userscript:

document.getElementById('sidebar').style.cssText = 'display: none; min-height: 525px;';
document.getElementById('content').style.marginLeft = '13px';

…or just use unsafeWindow.Layout.sidebar.hide(); (probably combined with the unsafeWindow polyfill).