If you know the dimensions of the inner div you can use CSS.
#outer {
position: relative;
}
#inner {
position: absolute;
width: 200px;
height: 200px;
left: 50%;
top: 50%;
margin-top: -100px;
margin-left: -100px;
}
There are other options using display: table-cell and vertical-align: middle etc.
Other options include JavaScript to dynamically determine the dimensions of the inner div and set the negative margins like above.
