On one of my projects I was trying to figure out how to center my footer div. It was after all the content, but I wanted to center it with the width of the center content and not overlap the sidebars.
Took me quite a while to figure it out so here it is:
.footer{
position:fixed;
bottom:0;
left:50%;
margin-left:-200px; /*negative half the width */
background:red;
width:400px;
height:100px;
z-index:999;
}