Because you can't always get a model when you need one.

Or maybe because you're bothered by all that pesky skin covering your model.

Posmania: "Posemaniacs is a site offering 3D, rotatable figures in a variety of poses for drawing. It has a program that chooses random poses and gives you a time limit to draw them and a perspective editor that makes guidelines for one-point perspective.

Though most of the site is in Japanese, the Google translation is pretty understandable.

from Metafilter.

Also recommended are Practical Anatomy and if you perfer your anatomy more abstracted here's a great collection of sculptures.