Visit PoseManiacs.com:
[link]They have
nearly every pose you could want for your reference pictures!
The models are 3D, so when looking at a certain pose you like, it's even possible to rotate the image along a 360-degree horizontal radius to view it from all angles.
One of my favorite parts about this is that the models are skinless, so you're allowed to see the complex bone and muscular systems beneath. I think it's an important key to accuracy.
Apart from static viewing images, it also has a feature called "30-second drawing", which switches images every 30 seconds (or whatever time interval you choose), forcing you to learn how to assess shapes and forms more quickly and render them faster. You can actually find examples of this exercise on YouTube, and it's pretty interesting to watch. Try it out.
My only complaint is that I can't find a site with a reliable cache of quality stock photo models for me to be inspired from, especially in terms of headshots and facial-structure. But that's a totally separate problem and has nothing to do with posemaniacs.
Even if you draw only occasionally, I think you'll find this a great help.
Enjoy!