Here is the creator’s story . . .
I'm Tomas Pettersson, and I created this application for my own enjoyment over the past six months (starting at the end of November 2009). This is an unpaid hobby project, but it has essentially consumed all my waking time during this period.
I don't want to demand any payment for the application, but of course I will appreciate a donation if you feel like it is valuable to you. Take a moment to consider what it would be worth in your particular case.
A pizza? A movie ticket? A day's salary?
Please do not consider donations as funding for future development. I will most likely take some more time to fix problems and improve various aspects, but I do not view this as my career. You can't "buy more features" (though I won't stop you from trying).
Sculptris will stay small and focused at its main task, which is sculpting and painting models. There is no point in expanding it to cover all the things advanced users can do in ZBrush, Blender, Maya and all the other big shot apps.