Apps: Notification LED simulator on buttons. Nokia sleepingscreen. Nokia situations.
We can use the light of the navigation buttons to have a notification LED on all phones. In fact, on AMOLED phones (like Lumia 800) we can use the screen for the same. (please port the app "nokia sleepingscreen" from symbian).
This is available in the Windows Phone Store for Nokia devices only. http://www.windowsphone.com/en-us/store/app/display/b08997ca-60ab-4dce-b088-f92e9c7994f3