The Calculus and Mathematica program at the University of Illinois and at Ohio State University WSU has IDEA , Internet differential equations activities. Some nice Java applets are being written by Jim Martino at Johns Hopkins University. Webwork , designed in the math department of the University of Rochester, offers immediate grading of homework and exams, online. Johns Hopkins uses this system in some courses. The primary product of this effort is a set of interactive, web-based learning modules.
Mathematical visualization is a burgeoning field.
Many links can be found on Dick Palais's Mathematical Exploratorium . The production of web material is a form of publishing, and is saleable. MathSoft offers web courses in science and math through first year calculus. It incorporates material from CCP . Theorist Interactive, based a little further away, in Harvard Square, produces a product called MathLive which looks like a pretty good mathematical communication tool.