You could use a linear-gradient.  Let's say the parent div had a background image, and you needed a div to sit on top of that with a gray background and a dog-eared left corner.  You could do something like this:

    .parent-div { background: url('/image.jpg'); }
    .child-div { 
       background: #333;
       background: linear-gradient(135deg, transparent 30px, #333 0);
    }

http://css-tricks.com/css3-gradients/

http://lea.verou.me/2011/03/beveled-corners-negative-border-radius-with-css3-gradients/