Traditional Media and Illustration
Will Eisner's Comics & Sequential Art
This is an excellent book. It's not so much for figure reference, but more about the design side of creating comics, which can be helpful to illustrators in general. There are parts about expressions and viewing angles of figures.
Figure Drawing Classes
If you want to expand your abilities in drawing the human figure, taking figure drawing (or life drawing) classes can really help, especially if you take classes at a college or university known for visual arts. When I studied graphic design in college, we were required to take two Drawing courses where we learned the basics of figure drawing using the box method and applied this knowledge to drawing live models. There were additional electives in Illustration and Life Drawing. I also took one of the Illustration courses, which expanded on what we learned in Drawing 1 and 2, and required the use of various traditional media (pencil, pen and ink, and scratch board).
How to Draw Manga: Male Characters
Even if you don't particularly want to work in a cartooning or manga-influenced style, I've found some good points in here that apply to drawing male figures in general. There's also mention of the differences between male and female anatomy (aside from the obvious). As a female illustrator, I've found this very useful.
Temple of the Seven Golden Camels
Mark Kennedy makes very detailed and insightful posts about illustration, especially figure drawing. The majority of his posts are instructional.
Using Photographic Reference and Live Models
Once you know the basics of figure drawing, practice is what will help you improve the most. Note that you do not need nude models to practice, but it does help when you're starting out if you can really see the shape of your model. Models in underwear or form-fitting swimsuits (no shorts!) are fine if you're uncomfortable with using nude reference material. From there, progress to fully clothed models, and you will have a better understanding of how the parts of the body fit together underneath the clothing. See the list below for places to acquire photographic reference.
Photographic Reference and Stock Photography
Warning: a lot of these include nude photography and are thus considered not safe for work. Always check the policies of those who provide photos before using them. Some of them have specifications about how their photos can be used. If in doubt, contact the photographer or model. In the case of stocky photography houses, most require payment to use their images. Many individual models and photographers will be open to allowing you to use their provided photos for pose reference, but you should still politely request permission if it isn't explicitly stated.
Art Dibujar's deviantART gallery
This photographer has a wealth of excellent photos in his gallery of adult males (nude and clothed). There are lots of interesting poses here. Note: you may need to have a deviantART account and be 18+ to view the nude photos.
Corbis Stock Photography
Commercial stock images. Multiple options to help you refine searches.
darkprophecy's deviantART gallery
This gallery includes photos of the gallery owner (his face is not visible) as well as other models. There are nude shots here, but nothing revealing.
David C's deviantART gallery
Male and female models, with a variety of clothed and nude subjects. There are also some beautiful landscapes here. Note: you may need to have a deviantART account and be 18+ to view the nude photos.
Female Anatomy for Artists
Female models, primarily nude. A great variety of poses. A pay site, but there are some free samples available so that you can see the quality. Members can request poses.
Gimp Savvy Public Domain Images
Free, public domain images available. Most of the images are from: National Aeronautics and Space Administration (NASA), the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Fish and Wildlife Service (FWS). Their photo sources have separate conditions for their use, so you will need to go off site to ensure that your intended use is approved (mostly this applies to people in photos if they're recognizable).
Human Anatomy for Artists
This has a wealth of excellent photos to use for illustrative reference. Primarily nude male models. A pay site, but there are some free samples available so that you can see the quality. Members can request poses.
Commercial stock images, videos, and Flash files. Subscription or pay as you go. Search low resolution images free.
Free, high resolution stock photography.
Free stock photos. Browse by category or use the site search.
Free photo resources. Mostly objects, scenery, and animals.
Female and male options. Virtual models can be viewed from different angles in the same pose. Additional features available if you sign up.