Wikipedia contains an encyclopedia entry with a simple overview. There is a link to the Axiom home page and links to all of the Axiom books.
Github is a master site for distributing releases. It is updated every two months. It contains the sources used to build the various download versions of the system.
git clone git://github.com/daly/axiom.git
Sourceforge is a development site for Axiom code. Daily changes are reflected in this git repository. You can download from here by typing:
git clone git://axiom.git.sourceforge.net/gitroot/axiom/axiom
Savannah is a development site for Axiom code. Daily changes are reflected in this git repository. You can download from here by typing:
git clone git://git.savannah.nongnu.org/axiom.git
Axiom is mentioned in Alasdair's Musings blog.