It'll be hard not to hum the Righteous Brothers and imagine a scene with Demi Moore and Patrick Swayze after using Sculptris (free), a 3D graphics program that simulates working with real clay. The innovative and extremely user-friendly interface allows you to select real-world commands like pinching, grabbing, flattening and smoothing, and apply them to a clay ball. You can choose symmetry across any axis, create wireframes, and other advanced tools to get your clay sculpture to look exactly how you want. You can even add spheres to the object for massive customization, or change the texture of the object. Ever want to sculpt in liquid metal? Now you can.
Pixologic designed Sculptris to have excellent compatibility with ZBrush (also from Pixologic), a painting application. Images can be exported directly from Sculptris to ZBrush using the GoZ tool, so after you sculpt your creation, you can refine it further using tools that only ZBrush has.
Unfortunately, Sculptris only supports two formats for exporting: ZBrush's, and Wavefront's OBJ files. That's not necessarily a drawback, though, as the OBJ format is standardized among 3D apps, including Poser, which I reviewed favorably. You'll be able to open the 3D object back up in 3D Studio Max or a similar 3D graphics program easily. You also may be irked by the fact that Sculptris doesn't have traditional Windows menus (not like Mac either; more like a series of icons and that's it); you'll have to hunt for the icons and tooltips to figure out how to save, load and export, and there's no About or Check for Updates or even Help.
For those who want to simulate clay sculpting in a 3D environment, or those who want to mess around with a free graphics application that's fun to use and different than everyone else's, give Sculptris a try. You may not be able to take your hands off it.