Does anyone know a good replacement for SAI Painter on a Macintosh?

I have recently decided to delve into the realm of digital painting and being rather short in the cash department i don't want to buy photoshop. I have heard that SAI painter is a good tool for artists, however, I am running OSX 10.5.7 and as SAI is a windows only program, I am wondering whether anyone knows a good alternative for Macs