I rarely share links here, but every now and then I find something I like and want to share. So here is a link for your enjoyment.
Google Body by Google Labs http://bodybrowser.googlelabs.com
This one is fun. It is an online anatomy guide by Google that show the forms in three dimensions. Its not really designed for artists, but it proves very useful in seeing how the muscles fit within the body. What I like is I can rotate the body and see the shapes of the muscles and bones from all sides. I don't know how accurate it is but looks pretty good to me.