So i've had Zbrush for a while but never felt comfortable using it. It has a funky UI and some weird workflow issues, but after a few hours of sitting and playing I think I've overcome the initial bumps and find that the program is actually pretty simple to use.
Zbrush is a 2.5d/3D painting/sculpting program. With it, you have the ability to carve, scrape, push and pull your 3d model into shape. It's a lot of fun.
Here are some pics of a sculpt I'm working on based on my Samurai character from a few post ago. The base mesh was modeled and optimized for a game engine, so it has a low poly count. I imported the base mesh into zbrush and divided the mesh up to 2million polys. The original count was in the neighborhood of 1000 polys.
Started with this mesh modeled in Lightwave 3D.
Wax Shader in Zbrush
Wet Clay Shader