Source downloads:
https://ftp.gnu.org/gnu/octave/
Windows binaries, with source code:
https://ftp.gnu.org/gnu/octave/windows/
Earlier screenshot of the GUI, but very similar to the current version:
http://www.walkingrandomly.com/images/octave/octave_GUI_38.p...
The Windows binaries are partially due to commercial sponsorship. We are still selling more commercial support, and we are working on providing even more commercial options:
http://www.gnu.org/software/octave/commercial-support.html
Sorry, no Mac OS X binaries at this point. It has proven to be a much more difficult target than Windows, due to the impossibility of cross-compiling into it, plus several idiosyncracies in Apple’s libraries. If you are comfortable with a CLI tool, Octave 4.0 is already in homebrew, which seems like the best option at the moment. If I could dedicate myself to this problem, I am confident I could put quite a dent into it.
If you are an Octave enthusiast, come to OctConf 2015 in Germany this September!