ncExploreMap is an addon that lets you see unexplored parts of the map as if you have explored them. This is a recreation of MozzFullWorldmap, just a bit stripped down. Also, this addon deletes any data from the database which you have explored, thus reducing memory usage. Goes from ~100kb on level 1 to ~5kb on level 85 with everything explored.
Install & go! Toggle the overlay by right clicking the world map close button.
Update for cata! Also changed toggle system to something more smooth (right click on close button).