If you want to create a "log out" button for users, here is the quickest way.

Create a new menu of “Link - Url” type in one of your menus.

Then, set its access level to "Registered" and give it any name you want like “Logout.”   Then, for the "link" or "url" use one of the following:

Joomla 1.0.xx: In the “Link” text box type the following “index.php?option=logout”.

Joomla 1.5: In the “Link” text box type the following “index.php?option=com_user&task=logout&return=Lw”.

That’s it.  You now have a "log out" button that only appears to registered users, and it is just one click.

But, you say, doesn't the Joomla "User Menu" come with a logout button?

Yes it does, but it requires you to confirm that you want to logout.  This is a one click solution!