Here are a bunch of artists who create (or created) beautiful, mathematical art:
Josh Millard – joshmillard.com and @joshmillard – creates amazing geometric paintings, often based on the Menger Sponge, and writes up his process on his website. A fellow Portlander! Check out his gallery here.
Wassily Kandinsky, who was painting geometric art back while you were cuddling a Cabbage Patch1. Check out his Circles in a Circle or On White II as classic mathy art. Lots more linked from his Wikipedia page.
Of course, M.C. Escher is a great mathematical artist example. See posts about his work here.
Any other examples? Hit me up: firstname.lastname@example.org!