Minecraft Skin Viewer is a small tool that will allow you to visualize various Minecraft skins in 3D.
To find skins, all you need to do is browse by username or upload your own file. The developer claims that the best thing about the tool is the ability to play with the position of your model by dragging and dropping, so you can pose it just how you like. You'll also be able to visualize the skins in movement.
If you use Minecraft Skin Viewer, you'll also be able to change the background, and save images of the skins for later.