From Stephen Wolfram, creator of Mathematica—"the world's most powerful mathematical software system"—comes The Wolfram Demonstrations Project.  The Wolfram Demonstrations Project is an "open-code resource that uses dynamic computation [i.e. Mathematica] to illuminate concepts in science, technology, mathematics, art,  finance, and a remarkable range of other fields.” Although Wolfram's most recent version of his computational software (Mathematica 6) is  required to author, modify, and publish Demonstration source code,  web previews of the Demonstrations are available online.  Alternatively, the free Mathematica Player allows anyone (with or without Mathematica 6) to "download a live version with active controls.”