Welcome to Sage, the open-source alternative to Magma, Maple, Mathematica, and MATLAB!
If you have come here to learn about the Sage program, these links will give you a good overview:
http://www.sagemath.org/tour.html Sage Tour
http://www.sagemath.org/library-stories.html Sage Testimonials
http://www.sagemath.org/library-why.html Why Sage?
If you are interested in development, the following links will help:
http://www.sagemath.org/doc/faq/faq-contribute.html FAQ: Contributing to Sage
http://www.sagemath.org/development.html Info for Developers
http://www.liafa.jussieu.fr/~labbe/Sage/how-to-contribute/ How to Contribute to Sage