1. Human Anatomy for Artists is a paid membership site that features a huge selection of professional hi-res photos of a wide variety of models both nude and in clothes. They offer a selection of great free samples: DOWNLOAD HERE which should give you a taste of what to expect. If you frequently use photo reference I’d wager this site is worth the membership cost.
2. Like the link above, this site is by the same creators but focuses on female anatomy alone. You will find all sort of different body types and shots from all angles. Download their free samples here: DOWNLOAD
3. Another site by the same creators as the two above. This one focused on anatomy for comic book artists. You can find great action shots in high quality and from all angles. The images are hi-res and very professional. View their free samples here: DOWNLOAD
4. Marcus J. Ranum is a bit of a legend on DeviantArt. His artistic nude photography is very popular but also creates some controversial debates. Marcus J. Ranum is a fantastic photographer and have been so kind as to release a wide variety of stock images free for use as long as you credit him.
The photos are mainly focused on females but there is also a great selection of guys in costume and hands. You can find his personal Deviant art site here: LINK
5. The Eirian-stock on Deviant Art features a wide variety of costume photography of both females and males. You can also find photo reference for backgrounds, animals and textures. I highly recommend checking the site out.
6. Pose Maniac is a site focused on human anatomy reference. The models are created in a 3D program so you get a large variety of angles and poses and as a added bonus: You can to see the muscle systems. Great for studying from.
7. Zen Textures is a texture only site which can be very useful to artists who like to use textures in their work. The textures are hi-res.
8. Photos4artists have a wide selection of photo references free for all artists to use. The references are mainly landscape and city setting photography but there is also a gallery of still life references. Really good selection of photos to do studies from.
I hope this selection was useful to you! Have fun creating.