Making a site's menu accessible has two basic principles.

There are multiple ways to implement menus accessibly, some of which may require additional work. Menu functionality can be broken down into two categories, managed focus and non-managed focus.

The principal difference is in determining how keyboard events are going to be handled. Non-managed means the browser takes care of controlling the flow of keyboard focus through the menu, and managed means you are responsible for controlling the flow of the keyboard focus through the menu.

Non-Managed Focus Menus

The browser might provide all of the functionality you need in order to successfully navigate your menu. The browser already makes the tab and shift-tab keys move to the next and previous links respectively. If your menu system can be navigated using only those two key combinations, in conjunction with pressing return to activate a link, then no additional work is needed. The user will simply see the menu system as a linear collection of links in a nested list that they can move backward and forward through. In this case the relationship that a menu item has with its siblings and parent and child menu items is determined through the list nesting.

A popular example of a non-managed focus menu is the jQuery-based Superfish.

Another menu example is drop-down menu from Simply Accessible.

Managed Focus Menus

If you want to use the arrow keys to navigate you menu and submenu, you must trap the keyboard events and translate those events into moving the focus through the menu system. The browser will not automatically translate the down arrow as meaning to go to the next menu item. You must handle that action yourself. By default, if you press the down arrow, the browser will simply try to scroll the page down.

By utilizing the arrow keys, you could form a more complex structure and provide some additional semantics to your menu so the user would know if a particular item was a sub-menu item of another item. In other words, you could begin to describe the structure of your menu. For example, the left and right arrows could move across the top level menu items in a horizontal menu, and the down arrows could open a submenu beneath each menu item. Using this technique also requires using ARIA attributes to describe the relationships.

A popular managed focus menu is from jQueryUI.

Another managed focus menu that is also accessible is from the Open Ajax Alliance

Menus That Contain More Than Links

All of the above are good techniques if you have simple menus and submenus that only contain links. Some menus, often referred to as "Mega Menus" contain many other types of objects like images, headings, and paragraphs. These can be very difficult to make fully accessible. You could make the case that these types of menus have moved beyond simple menus and are more in the realm of tabs and tabpanels.

ARIA techniques can help to make menus with submenus more accessible.

Further Reading