SAGE is an open source math software package/distribution/framework/environment/playground/laboratory.
It is developed by William Stein and followers at the University of Washington. Pretty young (the software) but on it's way to compete with all the other big M*s (Mathematica, Maple, Magma, Matlab, ...).
Long term goals are targeted in integrating all the small and highly specialized open source packages inside a common package - called "the car".
It's unique feature is the interface, which is simply a dynamic website served by SAGE.
So it could be distributed easily across many computers without installing and simple collaboration with other users possible, too - just try here!
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