// alternates odd - even post class on post_class();
add_filter ( 'post_class' , 'sfn_alternate_post_class' );
global $current_class;
$current_class = 'odd';

function sfn_alternate_post_class ( $classes ) { 
   global $current_class;
   $classes[] = $current_class;

   $current_class = ($current_class == 'odd') ? 'even' : 'odd';

   return $classes;
}

Alternate post_class();

PHP