Internet Explorer conditionnal comments

<!--[if IE]>
  	<link href="ie.css" rel="stylesheet" type="text/css" />
<![endif]-->

You can also target only a certain version of IE:
<!--[if IE6]>
  	<link href="ie.css" rel="stylesheet" type="text/css" />
<![endif]-->

Internet Explorer hacks

.class {
  width:200px; /* All browsers */
  *width:250px; /* IE */
  _width:300px; /* IE6 */
  .width:200px; /* IE7 */
}

Targeting Opera only

@media all and (min-width: 0px){
    .classname {}
}

Targeting Safari only

html:lang(en)>body  .classname {}

Targeting Google Chrome only

body:nth-of-type(1) p{
   color: #333333;
}

IE6 crash

<style>*{position:relative}</style><table><input></table>