Salowe and Warme developed a program for computing exact rectilinear Steiner minimal trees, which should be capable of handling up to 30 points routinely. It is available by anonymous ftp from ftp.cs.virginia.edu in pub/french/salowe/newsteiner.tar.Z. The program is described in the paper "Thirty-Five-Point Rectilinear Steiner Minimal Trees in a Day", in Networks: An International Journal , volume 25, 1995.

Also available is another Steiner tree program from Virgina, by Gabriel Robins and Tong-Tong Zhang. This one produces a good heuristic solution.