Regard3D is really good for free software and is available on Mac. I'm just not sure if it uses GPUs because from what I remember it seemed a bit slower than other software.
As far as I know, the best free software available that does not require CUDA is regard3d. It's obviously not as fast as CUDA enabled software, but I really like it a lot. If you have the money and CUDA hardware though, I'd use 3DF Zephyr instead.