Here is what Wikipedia says about the free Sage software:

http://en.wikipedia.org/wiki/Sage_(mathematics_software)

You can use an online version of  Sage for free at

http://www.sagenb.org/ sagenb

Here is the Sage documentation front page, with links to tutorials, thematic tutorials, frequently asked questions, etc.

http://sagemath.org/doc/

Here is a  blog post with a good overview about Sage:

http://georgm.blogspot.com/2007/03/sage-computer-algebra-system.html