GNU Octave
   High-level language, primarily intended for numerical computations that is mostly compatible with Matlab. History, news, documentation, and downloads.
   http://www.octave.org/

GNU Octave Repository
   Central location for custom scripts, functions and extensions to GNU Octave.
   http://octave.sourceforge.net/

Matlab/Octave Compatibility Packages
   Collection of matlab compatible functions for use in Octave.
   http://users.powernet.co.uk/kienzle/octave/

Octaviz
   3D visualization system for Octave. Description, news, and download.
   http://octaviz.sourceforge.net/

Wikipedia: GNU Octave
   Encylopedia article about the open source software product.
   http://en.wikipedia.org/wiki/GNU_Octave