If you're looking for something free give Blender a try. There are also trial versions of the more powerful applications like Maya, Studio Tools, and Lightwave available. For something easy to pick up and use right away try the demo of Rhino3D. SketchUp is a nice entry level 3D tool to play around with as well.
By the way - Blender is available on many platforms - Maya on Mac, Windows, and Linux - Lightwave on Mac and Windows - and the rest on Windows only.