throttle.js

// from underscore.js
var throttle = function(func, wait) {
  var context, args, timeout, result;
  var previous = 0;
  var later = function() {
    previous = new Date;
    timeout = null;
    result = func.apply(context, args);
  };
  return function() {
    var now = new Date;
    var remaining = wait - (now - previous);
    context = this;
    args = arguments;
    if (remaining <= 0) {
      clearTimeout(timeout);
      timeout = null;
      previous = now;
      result = func.apply(context, args);
    } else if (!timeout) {
      timeout = setTimeout(later, remaining);
    }
    return result;
  };
};