Just a small fun thing — if used as a bookmarklet (crunched with http://ted.mielczarek.org/code/mozilla/bookmarklet.html), creates a circular hole in the current page that you can use to peek at html code beneath. Chrome/Safari only.

htmlpeek.js

var init = function($) {
    $("<style type='text/css'></style>").text('.cx { background: #f5f2f0; color: black; }' +
        ' .cx .tag { color: #999; }' +
        ' .cx .tag .title, .cx .css .attribute { color: #905; }' + 
        ' .cx .tag .attribute, .cx .string, .cx .css .tag, .cx .css .class { color: #690; }' +
        ' .cx .tag .value, .cx .keyword { color: #07a; }' +
        ' .cx { font-family: Consolas, Courier New; position: absolute; z-index: 50000; left: 0; top: 0; right: 0; white-space: pre; }')
        .appendTo($('head'));        
 
    var source = $('<code></code>').text(document.documentElement.outerHTML)
                                   .addClass('cx')
                                   .hide()
                                   .appendTo($('body'));
 
    var centerX = 0;
    var centerY = 0;
    var radius = 100;
    var updateMask = function() {
        source[0].style.webkitMask = 'url(\'data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" width="' + 2*radius + '" height="' + 2*radius + '"><circle cx="50%" cy="50%" r="' + radius + '" fill="red" /></svg>\') no-repeat ' + (centerX - radius) + 'px ' + (centerY - radius) + 'px';
    };
 
    updateMask();
    source.show();
 
    $(document).mousemove(function(e) {
        centerX = e.pageX;
        centerY = e.pageY;
        updateMask();
    });
 
    $(document).mousewheel(function(e, delta) {
        e.preventDefault();
        radius += delta * 3;
        updateMask();
    });
 
    hljs.highlightBlock(source[0]);
};
 
var require = function(condition, url, callback) {
    if (condition) {
    	callback();
    	return;
    }
    	
    var done = false;
    var script = document.createElement("script");
    script.src = url;
    script.onload = script.onreadystatechange = function() {
        if (!done && (!this.readyState || this.readyState == "loaded" || this.readyState == "complete")) {
            done = true;
            callback();
        }
    };
    document.getElementsByTagName("head")[0].appendChild(script);
};
 
require(window.jQuery !== undefined, '//ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js', function() {
    require(window.jQuery.fn.mousewheel !== undefined, '//cdnjs.cloudflare.com/ajax/libs/jquery-mousewheel/3.0.6/jquery.mousewheel.min.js', function() {
        require(window.hljs !== undefined, '//cdnjs.cloudflare.com/ajax/libs/highlight.js/7.3/highlight.min.js', function() {
            init(window.jQuery);
        });
    });
});