I was looking for reference images for facial expressions and ran across a great site filled with not only reference images but a great deal of instruction as well.
The images and explanations can be found on www.cedarseed.com, ran by Joumana Medlej, a talented illustrator. The information seems to be primarily geared for illustrators, but is of great use for modelers and animators as well. Many of the images show major muscle groups that would help modelers and animators better plan their characters.
Not only does he have extensive information on humans, but he also has quite a few animal and bird references as well.
I encourage you to check it out.