Primes, sequences and modular arithmetic — properties of integers turned into form and pattern.
The site of a Japanese artist known for the "Star Cage". It presents bamboo sculptures, benches, an origami turbine and other works based on Fibonacci numbers and quasicrystal geometry, along with papers and Fibonacci-based music. The site design is old-fashioned, but it is still being updated.
A channel in which a mathematician presents famous problems and theorems in number theory and geometry, with a focus on visual proofs. Its many ways of showing properties of numbers as shapes are a good source of ideas for number-themed work.
A page extending the classic Ulam and Sacks prime spirals so that each number is drawn according to how many distinct prime factors it has. Every image comes with the Python script that produced it, so you can reproduce and modify them yourself.
A mathematics channel in which mathematicians and researchers each take up one topic about numbers. Number theory — primes, sequences, curious properties of numbers — is especially well covered, explained with hand-drawn diagrams. A good source of ideas for work built on numbers.
A site that walks through "number spirals" — integers wound so the perfect squares line up — and shows, chapter by chapter, the curves that primes and factors trace. The program used for the graphics can be downloaded, making it a good starting point for anyone building imagery from primes.