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. Perfect for brain breaks, wrapping up the day, indoor recess, or to analyze interesting strategies. Learn more...
21 Games for Paper and Pencil
Perfect for brain breaks, wrapping up the day, indoor recess, or to analyze interesting strategies. Learn more...
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!