Octave is a free, open source language for numerical computing that is mostly compatible with MATLAB. For years, the official Octave project has been strictly command-line only which puts many users off — particularly those who were used to the graphical user interface (GUI) of MATLAB. Unofficial GUIs have come and gone over the years but none of them were fully satisfactory for one reason or another.
As of the 3.8 release of Octave on 31st December 2013, this all changed when Octave started shipping with its own, official GUI. It is currently considered as ‘experimental’ by the Octave team and is obviously rough around the edges here and there but it is already quite usable.