Octave

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