How can I cite Octave?

Pointing to <http://www.octave.org> is good because that gives people a direct way to find out more. If citation of a URL is not allowed by a publisher or if you also want to point to a traditional reference then you can cite the Octave manual:

       @BOOK{eaton:2002
         author =     "John W. Eaton"
         title =      "GNU Octave Manual"
         publisher =  "Network Theory Limited"
         year =       "2002"
         isbn =       "0-9541617-2-6"
       }