Autodesk's Mudbox is a *digital sculpting and texturing tool for 3d artists. It was developed with an interface that closely resembles that of traditional 3d packages. So with any prior 3d experience Mudbox is easy to learn. Now that it is owned by Autodesk it is fully compatible with their other products Maya and 3dsMax, but it will export to other programs too. Mudbox has just expanded it's functionality to include texture painting and shading.
*Digital Sculpting is the process of using tools on extremely high polygon meshes. These polygon counts can exceed into the millions, and the process is very intuitive much like sculpting real clay.
Company: Autodesk Focus: Digital Sculpting and Texturing
2009 Mudbox features
DEMO VERSION
Mudbox is available for a 30 day non commercial trial. Download it here.
OFFICIAL INFORMATION
Designed by professional artists in the film, games, and design industries, Autodesk® Mudbox™ digital sculpting and texture painting software gives 3D modelers and texture artists the freedom to create without worrying about the technical details.
Autodesk Mudbox 2009 combines a highly intuitive user interface with a powerful creative toolset for creating ultra-realistic high-poly 3D models. Breaking the mold of traditional 3D modeling applications, Mudbox 2009 provides an organic brush-based 3D modeling experience that ignites the creative process.
SOFTWARE - ABOUT MUDBOX 
2009 Mudbox features 
DEMO VERSION 
WEBSITE