Sculptris: This is my favorite program ever, it sets the standard for great user interfaces and as a sculpting program its magnificent, loads of fun and VERY VERY easy to learn. Once you get good at this you will start to have this odd creeping feeling that you need more, thats where ZBrush comes in. Sculptris was written by one person; Dr. Peter, it was nearly complete when he was hired by PixoLogic and they bought Sculptris. Pixologic has added a few things, stabilized a few issues and made a direct link to and from ZBrush via something called GoZ. Sculptris is at version Alpha 6 with updates impatiently awaited for some time now, and hoped for in the almost instant future. Now would be good please. A great place to see what people are up to with Sculptris is at the Sculptris section of the ZBrush Central forum.
Zbrush: This is also my favorite program ever, I know that conflicts with what I said about Sculptris but its true. ZBrush is the best sculpting program out there and it can do things that no other software can even attempt. It has the best support of any software I have ever seen in the form of a website called ZBrushCentral.Com. ZBrush is capable of working with billions of polygons, has amazing painting features and is as close to perfect as anything I have ever seen.... Possibly better. Chances are if you have seen a movie, you have seen ZBrush work, its everywhere.
Blender: Blender is a complete 3D package for creating, animating and rendering 3D objects. With Blender if you know what you are doing, and have the resources, you can make entire movies. Blender is considered competition for things such as 3D Studio Max, Maya and the proprietary stuff large animation companies use, companies such as Pixar and Industrial Lights and magic for example. Blender is also FREE. Its open source and has a large community backing it up with programing, development and general user support. This is good cause its hard as hell to learn. The Blender Artists Forum is one of the better places to start.