While your computer has on-screen ways of notifying you new emails, IMs and other messages, they aren't much use when your display is asleep. DIY website Instructables show us how to create a small LED blinker for notifications.
This little hack makes use of a parallel port on your computer (which you might have to add yourself if your computer is new) and a small Greasemonkey script. All you need to do is stick some LEDs inside the open, female end of a parallel connector (as shown above), download the blinker application (Windows only), and load the user script into Firefox or Chrome. The user script was designed for Facebook notifications, telling the app to blink one LED for new messages and the other LED for other notifications.
While it was originally designed for Facebook notifications, you could probably tweak it to notify you of just about anything — perfect for when you're letting your computer's display sleep but are still in the room doing other things. Check out the video for a demonstration and hit the link for detailed instructions on how to put it together.
Facebook LED Blink on New Notifications and Messages [Instructables]