Software to display 3D surfaces What are some examples of software or online services that can display surfaces that are defined implicitly (for example, the sphere $x^2 + y^2 + z^2 = 1$)?  Please add an example of usage (if not obvious).  
Also, I'm looking for the following (if any):  


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*a possibility to draw many surfaces on the same sheet  

*to show cross-sections


Try these for algebraic surfaces:


*

*surf generates excellent images.

*surfer

*surfex
from http://www.algebraicsurface.net/.
