From http://bgrins.github.io/devtools-snippets/

ChromeSnippet

// jquerify.js
// https://github.com/bgrins/devtools-snippets
// Add jQuery to any page that does not have it already.

(function () {

  if ( !window.jQuery ) {
    var s = document.createElement('script');
    s.setAttribute('src', '//ajax.googleapis.com/ajax/libs/jquery/2.0.3/jquery.min.js');
    document.body.appendChild(s);
    console.log('jquery loaded!');
  }

})();

ChromeSnippet-Log

// log.js
// https://github.com/bgrins/devtools-snippets
// Adds a `log` function to window object.
// http://www.briangrinstead.com/blog/console-log-helper-function

(function() {

  window.log = Function.prototype.bind.call(console.log, console);

})();

DataURI

// dataurl.js
// https://github.com/bgrins/devtools-snippets
// Print out data URLs for all images / canvases on the page.

(function() {

  console.group("Data URLs");

  [].forEach.call(document.querySelectorAll("img"), function(i) {
    var c = document.createElement("canvas");
    var ctx = c.getContext("2d");
    c.width = i.width;
    c.height = i.height;

    try {
      ctx.drawImage(i, 0, 0);
      console.log(i, c.toDataURL());
    }
    catch(e) {
      console.log(i, "No Permission - try opening this image in a new tab and running the snippet again?", i.src);
    }
  });

  [].forEach.call(document.querySelectorAll("canvas"), function(c) {
    try {
      console.log(c, c.toDataURL());
    }
    catch(e) {
      console.log(c, "No Permission");
    }
  });

  console.groupEnd("Data URLs");

})();

Performance

// performance.js
// https://github.com/bgrins/devtools-snippets
// Print out window.performance information.
// https://developer.mozilla.org/en-US/docs/Navigation_timing

(function () {

  var t = window.performance.timing;
  var timings = [];

  timings.push({
    label: "Time Until Page Loaded",
    time: t.loadEventEnd - t.navigationStart + "ms"
  });
  timings.push({
    label: "Time Until DOMContentLoaded",
    time: t.domContentLoadedEventEnd - t.navigationStart + "ms"
  });
  timings.push({
    label: "Total Response Time",
    time: t.responseEnd - t.requestStart + "ms"
  });
  timings.push({
    label: "Connection",
    time: t.connectEnd - t.connectStart + "ms"
  });
  timings.push({
    label: "Response",
    time: t.responseEnd - t.responseStart + "ms"
  });
  timings.push({
    label: "Domain Lookup",
    time: t.domainLookupEnd - t.domainLookupStart + "ms"
  });
  timings.push({
    label: "Load Event",
    time: t.loadEventEnd - t.loadEventStart + "ms"
  });
  timings.push({
    label: "Unload Event",
    time: t.unloadEventEnd - t.unloadEventStart + "ms"
  });
  timings.push({
    label: "DOMContentLoaded Event",
    time: t.domContentLoadedEventEnd - t.domContentLoadedEventStart + "ms"
  });

  var navigation = window.performance.navigation;
  var navigationTypes = { };
  navigationTypes[navigation.TYPE_NAVIGATENEXT || 0] = "Navigation started by clicking on a link, or entering the URL in the user agent's address bar, or form submission.",
  navigationTypes[navigation.TYPE_RELOAD] = "Navigation through the reload operation or the location.reload() method.",
  navigationTypes[navigation.TYPE_BACK_FORWARD] = "Navigation through a history traversal operation.",
  navigationTypes[navigation.TYPE_UNDEFINED] = "Navigation type is undefined.",

  console.group("window.performance");

  console.log(window.performance);

  console.group("Navigation Information");
  console.log(navigationTypes[navigation.type]);
  console.log("Number of redirects that have taken place: ", navigation.redirectCount)
  console.groupEnd("Navigation Information");

  console.group("Timing");
  console.log(window.performance.timing);
  console.table(timings);
  console.groupEnd("Timing");

  console.groupEnd("window.performance");

})();

QueryString

// querystringvalues.js
// https://github.com/bgrins/devtools-snippets
// Print out key/value pairs from querystring.

(function() {

  var url = location;
  var querystring = location.search.slice(1);
  var tab = querystring.split("&").map(function(qs) {
    return { "Key": qs.split("=")[0], "Value": qs.split("=")[1], "Pretty Value": decodeURIComponent(qs.split("=")[1]).replace(/\+/g," ") }
  });

  console.group("Querystring Values");
  console.log("URL: "+url+"\nQS:  "+querystring);
  console.table(tab);
  console.groupEnd("Querystring Values");

})();

ShowHeaders

// showheaders.js
// https://github.com/bgrins/devtools-snippets
// Print out response headers for current URL.

(function() {

  var request=new XMLHttpRequest();
  request.open('HEAD',window.location,false);
  request.send(null);

  var headers = request.getAllResponseHeaders();
  var tab = headers.split("\n").map(function(h) {
    return { "Key": h.split(": ")[0], "Value": h.split(": ")[1] }
  }).filter(function(h) { return h.Value !== undefined; });

  console.group("Request Headers");
  console.log(headers);
  console.table(tab);
  console.groupEnd("Request Headers");

})();