Support routine for adding W3C user timing events to a site. Includes some basic polyfill support for browsers that don't support user timing or navigation timing (though the start time for non-navigation timing support could be improved with IE < 9 to use IE's custom start event).

user-timing-rum.js

// Support routines for automatically reporting user timing for common analytics platforms
// Currently supports Google Analytics, Boomerang and SOASTA mPulse
// In the case of boomerang, you will need to map the event names you want reported
// to timer names (for mPulse these need to be custom0, custom1, etc) using a global variable:
// rumMapping = {'aft': 'custom0'};
(function() {
var wtt = function(n, t, b) {
  t = Math.round(t);
  if (t >= 0 && t < 3600000) {
    // Google Analytics
    if (!b && window['_gaq'])
      _gaq.push(['_trackTiming', 'UserTimings', n, t]);
      
    // Boomerang/mPulse
    if (b && window['rumMapping'] && window.rumMapping[n])
      BOOMR.plugins.RT.setTimer(window.rumMapping[n], t);
  }
};
utReportRUM = function(b){
  var m = window.performance.getEntriesByType("mark");
  var lm={};
  for (i = 0; i < m.length; i++) {
    g = 'usertiming';
    if (lm[g] == undefined || m[i].startTime > lm[g])
      lm[g] = m[i].startTime;
    p = m[i].name.match(/([^\.]+)\.([^\.]*)/);
    if (p && p.length > 2 &&
        (lm[p[1]] == undefined ||
         m[i].startTime > lm[p[1]]))
        lm[p[1]] = m[i].startTime;
  }
  for (g in lm)
    wtt(g, lm[g], b);
};
utOnLoad = function() {utReportRUM(false);};
if (window['addEventListener'])
    window.addEventListener('load', utOnLoad, false);   
else if (window['attachEvent'])
    window.attachEvent('onload', utOnLoad);  

// Boomerang/mPulse support
utSent = false;
BOOMR = window.BOOMR || {};
BOOMR.plugins = BOOMR.plugins || {};
BOOMR.plugins.UserTiming = {
  init: function(config) {BOOMR.subscribe('page_ready', function(){
    if (!utSent) {
      utReportRUM(true);
      utSent=true;
      BOOMR.sendBeacon();
    }
  });},
  is_complete: function() {return utSent;}
};
})();

user-timing.js

/*
Strategically place:

markUserTime('some event');

Through your code to get measurements for when various activities complete.  It 
will also generate timeline events so you can see them in Chrome's dev tools.  

A good use case is to add them inline to sections of your site that are 
above-the fold (right after the menu, right after the main story, etc) and also 
in the onload handler for any critical above-the-fold images.
*/
;(function () {
  var w = typeof window != 'undefined' ? window : exports,
      marks = [];
  w.performance || (w.performance = {});
  w.performance.now || (
    w.performance.now = performance.now || performance.webkitNow ||
                             performance.msNow || performance.mozNow);
  if (!w.performance.now){
    var s = Date.now ? Date.now() : +(new Date());
    if (performance.timing && performance.timing)
      s = performance.timing.navigationStart
    w.performance.now = (function(){
      var n = Date.now ? Date.now() : +(new Date());
      return n-s;
    });
  }
  w.performance.mark || (
  w.performance.mark =
    w.performance.webkitMark ? w.performance.webkitMark :
    (function (l) {
      marks.push({'name':l,'entryType':'mark','startTime':w.performance.now(),'duration':0});
    }));
  w.performance.getEntriesByType || (
  w.performance.getEntriesByType =
    w.performance.webkitGetEntriesByType ? w.performance.webkitGetEntriesByType :
    (function (t) {
      return t == 'mark' ? marks : undefined;
    }));
}());
window.markUserTime = function(l) {
  var raf = window['requestAnimationFrame'] ||
    (function(callback){setTimeout(callback, 0);});
  raf(function(){
    window.performance.mark(l);
    if (window['console'] && console['timeStamp'])
      console.timeStamp(l);
  });
};

@jamischarles, the lognormal one is stable. RT plugin is opensource. UT isn't so much a plugin, nothing really to opensource, though I'm thinking of merging it into RT.