About a year ago, I was inspired by the Numberphile video on
Prime Spirals to try the same concept with composite (not prime) numbers. I wrote a program in Processing that would draw and output an image of an Ulam Spiral (see video) where each pixel is a number and each number is brighter depending on how many divisors they have other than 1 or themselves. Prime numbers have a brightness of 0, square semi-prime numbers have a brightness of 1, non-square semi-prime numbers have a brightness of 2, twelve has a brightness of 4, and so on. I liked the result, and decided to render the spiral in 1080p for a desktop background.
The final result (click to enlarge):
Also I was using it as a desktop background at my new job a few days ago and my co-workers replaced it with an MLP background.