GNU Octave (http://www.gnu.org/software/octave/) is a free numerical computing environment largely compatible with Matlab. John W. Eaton began the project in 1992 and remains the primary maintainer. Development is quite active with more than 20 volunteers currently involved. Octave is also widely used in teaching, industry, and research. It has gained attention as the recommended tool for a number of distributed online courses such as Stanford's machine learning course (https://www.coursera.org/course/ml). GNU Octave has an extensive core of functions useful for computations in several fields, such as numerical analysis, optimisation, signal and image processing, and control theory. Its sister project, Octave Forge (http://octave.sourceforge.net/), helps in speeding up development, increasing user contributions, and testing code before it is included in GNU Octave's core set of functions. Recent and ongoing improvements include a GUI and just-in-time compilation capabilities, both of which started as GSoC projects.