Sculptris isn't the only 3D modelling program out there, but it is one of the easiest to use, even if you have no experience with sculpting or digital modelling. All you need to do is point and click with your mouse to work with the digital "clay" on the screen. It even has a cool symmetry feature, where you can sculpt something and have it appear symmetrically on the other side of your model — perfect if you're building, say, a human figure and don't want to draw that eye all over again. If you have a tablet PC, it even supports pressure sensitivity, so you can get really detailed with your creations. Hit the link to check it out.