What is a good anatomy website for an artist?

Poses, including muscles: http://www.posemaniacs.com/ (great references!)

Other stuff I'd suggest looking up a skeleton and a body with muscles. Try to understand how muscles bend and stuff and where bones connect so that you don't create impossible bendy arms or something. Then, all you have to do is observe. Observe real life and how people move around you. How people move when they walk, when they run. The expressions they make. What facial expressions are possible and which aren't. Quirks and stuff that most people do but they don't even notice (when sitting we don't sit upright, we sit a little leaned back, etc...). All these stuff is important to create a convincing, natural picture.

Good luck! :D

By studying the anatomy of a human being,like how muscles react when you move your arms and legs or by bending over or twisting you then know when you are drawing or sketching something about a human being the way to draw the outer skin and where to shadow to represent a certain muscle or where veins are on the hands and feet. By studying what is under the skin you then learn to draw what is outside covering the underneath parts.This allows you to draw realistically.