A forum for people with knowledge of the Bible in its original languages to discuss its manuscripts and textual history from the perspective of historic evangelical theology.
I think it depends on the complexity of your stemma.If it's simple enough, you can use a standard drawing package like Paint or Draw. Even Powerpoint (though not technically "free").If the stemma is more involved but there is no mixture or contamination (e.g., for Spencer et al. on TC some years ago), you can use various cladistics programs, many of which are free.You can also ask Peter Robinson or his team (who produced STEMMA) for what they use.
Talking of Peter Robinson and STEMMA, I looked on the web-page below with affiliations to Robinson and his associates, including D.C. Parker and Barbara Bordalejo. I did not find any information on particular software here but there are surely many useful things here for the "stemma-interested"http://www.textualscholarship.org/stemmatics/index.htmlApparently, Bordalejo has her own blog at: http://www.textualscholarship.org/blog/ Unfortunately, however, there is not much activity there.