Categorical Geometry - contains online books and research papers on categorical geometry and categorical algebra.
Dr Geo - free interactive geometry software avalaible in different languages.
Erich's Packing Center - packing geometric shapes into other geometric shapes: circles in triangles, circles in circles, and much more, plus combinatorial geometry problems.
Isard - downloadable, interactive geometry software package written in Smalltalk.
Leningrad Geometric School - a famous collective body of Russian geometers that rejected the methods of differential geometry.
Logical Art and the Art of Logic - learn about pentominoes and what they have to do with Escher, Penrose, logic, impossible objects, mathematics, and art.
Mathlab.com - Java applet providing virtual straightedge and compass for online interactive geometry.
Mrs. Glosser's Math Goodies - free educational web site featuring interactive math lessons with a problem-solving approach.
Native American Geometry - a physical, proportional geometry that originates from the simple circle. Art and mathematics seemlessly integrated in Nature's geometry, emphasizing multiple intelligences and multi-cultural instruction.
Peek - a program to interactively visualize arbitrary n-dimension polytopes through the use of cross-sections and projections.
Ptolemy's Ptools - where no kid has gone before in geometry. Use similar triangles to explore the world from your backyard and 3D worlds from your computer.
Taxicab Geometry - a nice, gentle introduction to non-Euclidean geometry.