Sculptris is a free 3D modeling application, that is unusual in that anyone can pick it up and play without any technical knowledge.
Incredibly, Sculptris doesn't require installation, and is surprisingly lightweight for a 3D modeling application. Each time you start, a ball of 'clay' appears in the center of the program. To the left is your toolkit, your digital 'hands' that will model the clay. Above are controls that allow you to tweak the tool size, strength and alter the material you see.
Holding the ALT key and the mouse allows you to rotate the ball, and move the camera. The mouse buttons alone activate your Sculptris tool. You can drag out clay to form shapes, or sculpt inwards. The right button generally does the opposite of the left, so if the left pushes in, the right will pull out. After a couple of minutes using Sculptris becomes second nature.
What Sculptris can't teach you is how to be an good artist, and while it's easy to pick up, you'll still need to spend time practicing, as creating a good piece takes a lot of effort.
Sculptris is an ideal way to get a feel for 3D modeling. It's easy to pick up, and the results are really limited only by your imagination.