One of the users on my site posted the following code yesterday: <img src="/logout">. This causes every user to log out when visiting the page that code is on. He suggested making a special page at that address that uses a form to log out.
While we're at that, a system similar to that at Tweakers.net might be made. There you can select the session you want to log out. In that way you can log out the session you started on work bur forgot to end and you can do it from your home computer.