You can use the_category if you are in the loop:
<?php the_category(', '); ?>
If not, then use this code:
<?php
$output="";
foreach((get_the_category()) as $category) {
if($category->name==$homecat) continue;
$category_id = get_cat_ID( $category->cat_name );
$category_link = get_category_link( $category_id );
if(!empty($output))
$output .= ', ';
$output .= '<span class="cat"><a href="'.$category_link.'">'.$category->cat_name.'</a></span>';
}
echo $output;
?>