Non-Euclidean Virtual Reality is a project by Vi Hart, Andrea Hawksley, and Henry Segerman, with help from many others. It's a virtual reality world that you can move around in the normal forwards-backwards, up-down, right-left directions, but rather than behaving like a euclidean 3D space, it is a hyperbolic space. This allows for things like four ninety degree angles fitting in a plane. One of my favorite applications of graphics is to demonstrate mathematical concepts, and this is next level immersion in a math space, that gives an intuition that would otherwise take a whole lotta reading and thinking to achieve.
You can try it out here, move around with arrow keys and WASD keys.
Here's a video:
And an image: