Pternary draws points, lines, and polygons on a triangular (ternary) graph. It has a graphical user interface written in Tkinter.
It requires Python 2.3 and Tkinter, which should come pre-installed on Mac OS X 10.4+
See the README page (on the right) for more. The latest version of the script is available under the Source tab - see changes.txt for what's new.