Simple Responsive Menu is a drop-in replacement for the standard Joomla vertical menu.
At wider screen widths, you should see no difference at all from the standard Joomla menu, but at narrower screen widths the menu switches to a select list. Simple Responsive Menu lets you configure the screen width where this switch takes place, and the position of the select list.
For small screens, it changes to a select list, which takes very little space when collapsed. When expanded, the list shows all the options.
Simple Responsive Menu Pro
Simple Responsive Menu Pro is an enhanced version that works in a very similar way but has a choice of great icons for the menu link, and uses formatted HTML for the mobile menu. It's not free, but it just looks better!
Simple Responsive Menu is completely free, with no restrictions on its use.