app/etc/modules/ModuleName_All.xml

<?xml version="1.0"?></pre>
<config>
<modules>
<ModuleName_Catalog>
<active>true</active>
<codePool>local</codePool>
</ModuleName_Catalog>
</modules>
</config>


app/code/local/ModuleName/Catalog/etc/config.xml

<?xml version="1.0"?>
<config>
<modules>
<ModuleName_Catalog>
<version>0.1.0</version>
</ModuleName_Catalog>
</modules>
<global>
<blocks>
<ModuleName_catalog>
<class>ModuleName_Catalog_Block</class>
</ModuleName_catalog>
</blocks>
<models>
<catalog_resource_eav_mysql4>
<rewrite>
<product_collection>
ModuleName_Catalog_Model_Resource_Eav_Mysql4_Product_Collection
</product_collection>
</rewrite>
</catalog_resource_eav_mysql4>
</models>
</global>
</config>


app/code/local/ModuleName/Catalog/Block/Product/Listcategories.php


<?php
class ModuleName_Catalog_Block_Product_Listcategories
extends Mage_Catalog_Block_Product_List{

//same function from Mage_Catalog_Block_Product_List
protected function _getProductCollection(){

// get simple product collection
$this->_productCollection = Mage::getModel('catalog/product')->getCollection();
$this->_productCollection->addAttributeToSelect('*');

// if we receive categories parameter we filter with our custom function addCategoriesFilter
// currently there's a addCategoryFilter, but only accepts 1 category
// if no param is passed, simply show everything
if($this->getCategories()!="")
$this->_productCollection->addCategoriesFilter($this->getCategories());

return $this->_productCollection;
}
}
?>



app/code/local/ModuleName/Catalog/Model/Resource/Eav/Mysql4/Product/Collection.php
<?php
class ModuleName_Catalog_Model_Resource_Eav_Mysql4_Product_Collection
extends Mage_Catalog_Model_Resource_Eav_Mysql4_Product_Collection{

public function addCategoriesFilter($categories){

$alias = 'cat_index';
$categoryCondition = $this->getConnection()->quoteInto(
$alias.'.product_id=e.entity_id AND '.$alias.'.store_id=? AND ',
$this->getStoreId()
);

$categoryCondition.= $alias.'.category_id IN ('.$categories.')';

$this->getSelect()->joinInner(
array($alias => $this->getTable('catalog/category_product_index')),
$categoryCondition,
array('position'=>'position')
);

$this->_categoryIndexJoined = true;
$this->_joinFields['position'] = array('table'=>$alias, 'field'=>'position' );

return $this;

}
}
?>


This is what I have tried to load in a cms page:

<p>{{block type="ModuleName_catalog/product_Listcategories"  categories="10,13,16"  template="catalog/product/list.phtml"}}</p>