Hm, I've heard of Zbrush, but I've only ever seen it used for sculpting 3D models.I'd also be interested in a tutorial on using it for 2D.
I use Graphicsgale and at times, Gimp.
Graphicsgale is very good for what you might call "Pixel art". It's nice and straight forward, and best of all: free. If you're just getting started with drawing characters, I recommend tracing over old sprites (Such as 2D Mario or Link etc.) to start off, it really helps you out.
Gimp is like a free version of Photoshop, with many of the same features.