This one is very useful for sites with multiple authors. The snippet only highlights the post author, as opposed to all of the authors on the site.
Insert this into your CSS file:

 
.commentlist .bypostauthor { 
/* -- Add a darker border - */ 
border: 1px #bbb solid; 
/* -- CSS3 Linear Gradient -- */ 
background:-webkit-gradient(linear, 0% 0%, 0% 100%, from(#f5f5f5), to(#ddd)); 
background:-moz-linear-gradient(0% 100% 90deg,#ddd, #f5f5f5); 
/* -- CSS3 Shadow -- */ 
-webkit-box-shadow:0px 0 50px #ddd; 
-moz-box-shadow:0px 0 50px #ddd; 
box-shadow:0px 0 50px #ddd; 
}