Bootstrap is a great CSS and Javascript framework. One of its brilliant features is the dropdown menu, which gets expanded when you click on it. In order to make it expand on mouse hover, you can just put this CSS inside your style sheet.

ul.nav li.dropdown:hover ul.dropdown-menu{
display: block;
}

/* if you want to hide the arrow */
a.menu:after, .dropdown-toggle:after {
content: none;
}

Source