Accordion Menu Lite is a free extension for Joomla!. And it supports 2 menu levels.
Using it for your site simply with two steps only: Installation and Assign a menu from Joomla menu system. With 4 options at configuration page:
- Menu Name: Select a menu from Joomla menu system
- Module Class Suffix: Assign a module template classufix for your module
- Remind Opened Menu: Remind opened sub menu when you click.
- Parent items link: Parent item is a menu link or not