A nice introduction to Sage on the CUNYMath Blog

SAGE is an open-source symbolic math computation software. Unless Octave or Matlab, it can process and computes objects such as equations, rings or fields, graphs and so forth.

For example, it can symbolically compute series, such as

sage: sum((1)/(n^4),n,1,Infinity)
1/90*pi^4

There is a very nice introduction on the CUNYMath Blog, that goes over a few of the basics. For a longer intro, there are tons of tutorial on the SageMath website. And for people wanting to learn calculus and/or Sage, the Smith College has a page on Calculus in Context.