fadeIn and fadeOut functions that work on elements or ids (no libs required)

fade.js

// TODO: IE support if I feel like it; make fade length directly configurable
(function() {
  var DELTA = 0.06,  // how much to change opacity each step
      DURATION = 40; // how long in ms to pause between steps
  function fadeFunc(direction, start, end) {
    return function(el) { 
      el = typeof(el) == 'object' ? el : document.getElementById(el);
      var step = function step() {
        var opacity = el.style.opacity;
        opacity = opacity === "" ? start : parseFloat(opacity);
        var next = opacity + DELTA*direction;
        if (Math.abs(next - end) < DELTA) {
          el.style.opacity = end; 
        } else {
          el.style.opacity = next;
          setTimeout(step, DURATION);
        }
      }();
    };
  }

  // generate fadeIn and fadeOut functions
  window.fadeIn = window.fadeIn || fadeFunc(1, 0, 1);
  window.fadeOut = window.fadeOut || fadeFunc(-1, 1, 0);
})();