This box is centered in most browsers, but not Internet Explorer.

#div1 {
      margin: 0 auto;
}

This box is centered in Internet Explorer, but only because its parent element is set to text-align: center.

#center_div2 {
      text-align: center;
}

#div2 {
      margin: 0 auto;
      text-align: left;
}

(Thanks to Christopher for pointing this out! He requested that I provide a link to StopIE.com.)

This box is centered by positioning.

#div3 {
      position: relative;
      left: 50%;
      margin-left: -30%;    /* half of #div3's width of 60% */
}