<div class="caja caja-sombra">
      <h2>Ejemplo 2</h2>
      <p>Sombra esquinas inferiores</p>
    </div>

CSS

.caja{
    width: 200px;
    height: 150px;
    background-color: #f4f4f4;
    border-radius: 5px; 

  border: 1px dotted #9C9C9C;
}
.caja-sombra{
    position: relative;
    float: left;
    padding: 1em;
    margin: 100px;
    box-shadow: 0 1px 4px rgba(0, 0, 0, 0.3), 0 0 40px rgba(0, 0, 0, 0.1) inset;
}
.caja-sombra:before, .caja-sombra:after{
    content: "";
    position: absolute;
    z-index: -2;
}
.caja-sombra:before{
    bottom: 15px;
    left: 10px;
    width: 50%;
    height: 20%;
    box-shadow: 0 15px 10px rgba(0, 0, 0, 0.7);
    -webkit-transform: rotate(-5deg);
    -moz-transform: rotate(-5deg);
    -ms-transform: rotate(-5deg);
    -o-transform: rotate(-5deg);
    transform: rotate(-5deg);
}
.caja-sombra:after{
    bottom: 15px;
    width: 50%;
    height: 20%;
    right: 10px;
    left: auto;
    box-shadow: 0 15px 10px rgba(0, 0, 0, 0.7);
    -webkit-transform: rotate(5deg);
    -moz-transform: rotate(5deg);
    -ms-transform: rotate(5deg);
    -o-transform: rotate(5deg);
    transform: rotate(5deg);
}