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
Here is the Sage documentation front page, with links to tutorials, thematic tutorials, frequently asked questions, etc.
Here is a blog post with a good overview about Sage:
http://georgm.blogspot.com/2007/03/sage-computer-algebra-system.html
[http://www.diveintopython.net/ Dive into Python] a free online book about learning the language used in Sage;
two pages on LISTS that look interesting: [http://www.diveintopython.net/native_data_types/mapping_lists.html] and [http://www.diveintopython.net/native_data_types/lists.html]
[http://docs.python.org/tut/ Python Tutorial] an online tutorial on the Python language.
Here is a Sage wiki:
http://wiki.sagemath.org/
Here are answers to some questions along the line “How do I construct … in Sage?””
http://sagemath.org/doc/constructions/index.html