In the author's words, “Prime Grid creates a simple, adjustable grid that plots prime numbers in a left-to-right, top-to-bottom layout. Use it to find interesting visual patterns, do some smart-person math stuff, or maybe find the secret code to the cosmos or whatever.”
What I found really cool about this very minimalistic grid is that I didn't expect any patterns to arise, but you can definitely see visual patterns when you play around with the grid dimensions. For example, if you set the grid to 205 x 209, what are those two wide empty spots along the main diagonal of the grid?! (I'm sure there is a good maths explanation, but it's not obvious to me what it is.)