Nothing beats drawing from life, but let's say you live out in the middle of nowhere and are sick of drawing yourself and your surroundings. Or maybe you need something during the week, between that weekend drawing co-op you like to attend. Perhaps you've tried the good 'ol internet search and come up with dinky or just plain weird images. Or you've tried drawing from magazines but the people in there have been airbrushed right out of reality and the lighting is flat so you struggle to understand form.
Try this, it's free. http://artists.pixelovely.com/
The lighting is pretty good, though some images are flat looking. You can choose from male or female models, clothed or not. You can focus on just hands or feet. You can study heads and expressions. Or you can study animals.
One word of warning: watch out for distortion. Some photos look like they have been corrected, some have not.