$block = Mage::getBlockSingleton('core/template');
$block = Mage::getBlockSingleton('custom_module/my_block');This code will retrieve the singleton instance of a custom block class called `my_block` that belongs to the `custom_module` package. This block can be used to render custom templates and widgets in Magento. The `Mage` class and the `getBlockSingleton` method are part of the Magento core library.