http://mathcs.slu.edu/escher/index.php/Math_and_the_Art_of_M._C._EscherMath and the Art of M. C. Escher, by Anneke Bart and Bryan Clair. Textbook (with explorations and exercises) for a course about symmetry as exhibited in the works of Escher.
www.geometrygames.org/KaleidoTile/index.html Interactive Windows and Macintosh program that lets the user design tilings on the plane, the sphere, and the hyperbolic plane. Spherical tilings can be realized as polyhedra.
demonstrations.wolfram.com/ComplementTiling/ Interactive Web program to make Escher-like tilings, with links to other demonstrations.
demonstrations.wolfram.com/TilingConstructor/ Free software for creating tilings, with a great many options; author Karl Scherer offers several related demonstrations, available from this page.
demonstrations.wolfram.com/PentagonTilings/ Free demonstration of all 14 known families of tilings by pentagons; the user can adjust angles, sides, and directions of repetition.
www.cgl.uwaterloo.ca/~csk/software/penrose/ Applet (online or downloadable) that lets you “warp” Penrose’s darts and kites to make tilings with other shapes of tiles.
www.tilingsearch.org Database of high-quality images of tilings, which allows searching by shapes and by numerous other criteria.
www.tess-elation.co.uk/ David Bailey’s World of Escherlike Tessellations, including essays, artwork, and explanations of Penrose tiles and other tessellations.
www.dbsullivanstudio.com/downloads/tutorials/tessellation_tutorial_part_1.pdf A tutorial on how to draw a tessellation with tiles of varying shapes. (There appears to be no Part 2 available.)
www.tesselmaniac.com/tess/TesselManiac.html Macintosh and Windows tessellation software (payware).
tesseUation.mfo/en/mfo/artists/17/Marjorie_Rke/ Site with some of Marjorie Rice’s artistic works created from pentagon tilings.
www.eschertiles.com/links.html Links to sites with Escher-inspired tilings and also to the official M. C. Escher site.
nlvm.usu.edu/en/nav/vlibrary.html Applets for tessellations and fractals.