Understanding 4D while living in a 3D world.
A stunning series of videos (freely available for download or online viewing) that teach you how to to visualize four dimensions. Titled Dimensions, these videos were created by a French professor of mathematics in collaboration with a graduate student and a Belgian artist.

What is truly amazing is that these videos are actually 2D (as all videos are). So in essence we are understanding the nature of the 4th dimension through representations that are actually 2 dimensional. I wonder if it would be easier to visualize the 4th dimension if we had holographic 3D representations that we could manipulate? Absolutely fantastic, none the less.