ZBrush is the original *digital sculpting tool, it also comes with a wide range of texturing and 2d paint features. It uses a unique interface paradigm because it grew out of a painting discipline, however it is an impressive 3d sculpting tool that can handle millions of polygons on screen at any time (so long as a computer can handle it). It's big community encourages artists to share their work and tips and sports some incredible artwork. Many tutorial videos can be found on the ZBrush site.
*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.
ZBrush is available for a 30 day non commercial trial. Download it here.
OFFICIAL INFORMATION
ZBrush 3.1 continues our mission of creating innovative and powerful tools for digital artists. ZMapper and Displacement Exporter have been ported for ZBrush 3.1 to allow more flexibility for exporting your sculpt. Using our Smart Resym technology, ZBrush 3.1 now includes Poseable Symmetry. Keep symmetry on your model at the click of a button. No matter the pose. Check out what its like to sculpt in the new ZBrush. Upgrade your copy of ZBrush now.
ZBrush runs on most modern Microsoft and Mac based computers. Though ZBrush will take advantage of all the bells and whistles of a newer machine. For full details go here.
PRICE GUIDE
These prices are dated from Oct 2008 ZBrush 30 day Trial: Free ZBrush: US $595.00
SOFTWARE - ABOUT ZBRUSH 
ZBRUSH Interviews Rick Baker - 6 time Academy Award Winner 
DEMO VERSION 
WEBSITE