underscore.assign.js

  /**
   *	Assign a value to the delimited key in the given object. The inverse of `_.lookup`
   *
   *	@example
   *
   *      var myObj = {};
   *
   *	  _.assign(myObj, 'foo.bar', 'baz'); // myObj = { foo: { bar: 'baz' }}
   *
   *	@param	{Object}  obj the object to assign to
   *	@param	{String}  key the key to assign to
   *	@param	{???}     value the value
   */
_.mixin({
  assign: function (obj, key, value) {
    var keys = key.split('.'),
        cur, ptr = obj;

    while ((cur = keys.shift()) && keys.length) {
      if (!_.isObject(ptr[cur])) {
        ptr[cur] = {};
      }
      ptr = ptr[cur];
    }
    ptr[cur] = value;
    return obj;
  }
});

underscore.filterBy.js

 /**
   *	Filter a collection based on the value of a specified key
   *	Requires _.lookup
   *
   *	@example
   *
   *	  var collection = [
   *		{ type: 'fruit', name: 'Apple' }
   *		{ type: 'vegetable', name: 'Sprouts' }
   *		{ type: 'fruit', name: 'Orange' }
   *	  ];
   *
   *	  _.filterBy(collection, 'type', 'fruit');
   *
   *	@param	{Array}	list  the collection to filter
   *	@param	{String}  key the key to compare
   *	@param	{???} value	the required value
   */
_.mixin({
  filterBy: function (list, key, value) {
    return _.filter(list, function (obj) {
      return _.lookup(obj, key) === value;
    });
  }
});

underscore.idle.js

  /**
   *	Defer a function while an action is taking place
   *
   *	@see http://blog.rjzaworski.com/2012/02/idling-with-underscore-js/
   *
   *	@example
   *
   *	  // `foobar` will not run until 2 seconds after the last time 
   *	  // it was called:
   *	  var foobar = _.idle(function() {
   *	    console.log('tic-toc');
   *	  }, 2000);
   *
   *	  foobar();
   *	@param	{Function}	code	to run eventually
   *	@param	{Number}	delay	time to delay after last function call
   */
_.mixin({
  idle: function(code, delay, context) {
    var handle;
    if (typeof(context) == 'undefined') {
      context = this;
    }
    return function() {
      if (handle) {
        window.clearTimeout(handle);
      }
      handle = window.setTimeout(_.bind(code, context), delay);
    }
  }
});

underscore.lookup.js

  /**
   *	Return the value corresponding to the key in the given object
   *
   *	@example
   *
   *      var myObj = {
   *	    foo: { bar: 'hello, world!' }
   *	  };
   *
   *	  _.lookup(myObj, 'foo.bar'); // "hello, world!"
   *
   *	@param	{Object}  obj the object containing the key
   *	@param	{String}  key the key to look up
   */
_.mixin({
  lookup: function (obj, key) {
    var type = typeof key, i = 0, length;
    if (type == "string" || type == "number") {
      key = ("" + key).replace(/\[(.*?)\]/g, function (m, key) { //handle case where [1] or ['xa'] may occur
          return "." + key.replace(/^["']|["']$/g, ""); //strip quotes at the start or end of the key
      }).split(".");
    }
    for (length = key.length; i < length; i++) {
      if (_.has(obj, key[i])) obj = obj[key[i]];
      else return void 0;
    }
    return obj;
  }
});

in underscore.lookup.js
there should be

for(; i < key.length; i++) { //...

and remove the l variable in declaration line, because we don't want to get the length of the string

Thanks for the catch @wesleycoder