To improve figure drawing and sketching skills, drawing gesture sketches is a great technique. Here's a little tool I made over a weeked to show a slideshow of stock images to practice.
Warning: Depending on your slideshow choices, some of these images may be NSFW. Please use them as intended, a sketching reference.
Trying to sketch human forms a bit more, I stumbled across artists.pixelovely.com's great tool for figure drawing. The idea is pretty simple: have a slideshow of appropriate subjects that changes often enough that you have enough time to get down the gesture and basic form but not enough to lose yourself in the details.
I loved the tool so much I wanted to have a go at doing the same thing with my own library of stock imagery (all of it properly cited), and maybe in the future building out the library a bit more. While mine is less feature rich, it does what I need it to. Being an experiment (and mainly for personal use), I haven't tested it in all browsers, but the current version of IE/FF/Chrome should be fine.