Much has been written about how the work of M.C. Escher was inspired by mathematics and has inspired mathematicians in turn. The relationship between math and art in the work of Escher is still the subject of analysis and discussion: in this month's Notices of the AMS, there is a very interesting article on the mathematical aspects of M.C. Escher's work by Doris Schattschneider.
As Schattschneider describes, the mathematician that alternately inspired and frustrated Escher in much of his work was Donald Coxeter. If you haven't read it yet, you should check out Siobhan Roberts's King of Infinite Space, a popular biography of Coxeter, which also discusses his relationship with Escher.
Way back in 2003, the hyperbolic tilings that Escher and Coxeter corresponded about were used for the Mathematical Awareness Month poster, and Douglas Dunham wrote an accompanying essay to explain how the image for the poster was created.
My own meager contribution: here are two little GSP activities (one here, the other here) inspired by some of Escher's plane tilings.