Auralin Viewport is an addon designed to reshape the game's WorldFrame.
This is not your run-of-the-mill bar creator. AVP doesn't create a black bar that covers the game. Instead, it makes a Viewport bar that changes the frame shape the game client renders the world in.
This essentially increases your viewport. If you have a standard monitor, put a viewport on the bottom or top of your screen, and you now have a widescreen view. The world's best players have been doing this for years, and now you can easily adjust these values yourself.
The beauty of this kind of add-on is that the space isn't simply wasted. You can put any addon you like over the empty space. This lets you neatly put your damage meters, chat, raid frames, etc., off your world frame so you can see more of Azeroth. You can also use Texturing addons over your empty bar.
Type /avp to bring up the options menu.
WARNING: Because this addon adjusts the world frame, saving and reloading between every change you make is highly recommended. Not doing so will result in stability issues and lead to the client crashing. This crash can occur quickly due to system memory overflow; you should save & reload immediately after making a change. So long as you press the save button after each change, you should avoid any issues. This is not a LUA error; it is a limitation of the memory the game has access to.