I just found out about this free program written by researchers at the Swiss Federal Institute of Technology in Zurich. It performs automatic retopology and also guided semi-automatic retopology. It was demonstrated at SIGGRAPH Asia 2015. You can download it from GitHub for free (the actual compiled executable app is there). The license there says that you can use it for any purpose (including commercial purposes), also see the license there for any further questions. I haven't had a chance to try it yet, but I am interested to see how it compares to 3D-Coat's Autopo or Zbrush's Zremesher.
Demo video:
https://www.youtube.com/watch?v=U6wtw6W4x3I
Download from GitHub here:
https://github.com/wjakob/instant-meshes
I will try to test it sometime this week if/when I get time. If you test it out, please post your results in this thread!