3D-XplorMath is a Mathematical Visualization program. The older original version, written in Pascal, runs only on on Macintosh computers, but there is also a newer cross-platform Java version, called 3D-XplorMath-J, that while still having fewer features and Exhibits, is currently under intensive development and is rapidly catching up with its older sister.
It presents itself as a gallery of interesting mathematical objects, ranging from planar and space curves to polyhedra and surfaces to ordinary and partial differential equations, and fractals.
Morever, the carefully chosen default parameters and viewing options may be changed by the user so that the gallery is turned into a experimental lab.
Every exhibit has its own online documentation with suggestions for how to explore it further.